Topographically generated fronts, very nearshore oceanography and the distribution of larval invertebrates and holoplankters

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1 JOURNAL OF PLANKTON RESEARCH j VOLUME 2 j NUMBER 1 j PAGES j 23 Topographically generated fronts, very nearshore oceanography and the distribution of larval invertebrates and holoplankters ALAN L. SHANKS, ANITA MCCULLOCH AND JESSICA MILLER OREGON INSTITUTE OF MARINE BIOLOGY, UNIVERSITY OF OREGON, PO BOX 389, CHARLESTON, OR 9742, USA CORRESPONDING AUTHOR: ashanks@oimb.uoregon.edu Foam lines oriented parallel to shore are common features of rocky shores. At times, the water coloration is different on either side of the foam lines, suggesting they are associated with fronts. We investigated the effect of shore-parallel foam lines and associated fronts on distributions of holo- and meroplankton. We performed CTD transects to describe the fronts and carried out vertical zooplankton tows to describe the distribution of zooplankton relative to the fronts. Fronts were within tens of meters of shore and were apparently generated by the interaction of coastal currents with local topography. We sampled four sites (three coves and one open coastal site), some of which were separated by only a few hundred meters. At each site we found shore-parallel foam lines and associated thermal fronts, but the characteristics of the fronts were different at three sites, suggesting that three different mechanisms were generating the fronts. At two coves, the foam line and front appeared to be due to the interaction of wind-driven currents from the north with coastal topography. At the third cove, the front appeared to be due to the expansion of solar-heated surface waters out of the cove. The foam line and front at the open coastal site appeared to be due to boundary mixing. At the coves, the distributions of holoplankters, meroplankters and phytoplankton were clearly altered by the presence of the fronts. At the open coastal site, the front had less effect on the distribution of zooplankton. The coastal ocean is the source of new recruits to the intertidal zone and an important source of food in the form of phytoplankton for filter feeders. We hypothesize that these very nearshore fronts may play an important role in structuring intertidal communities with which they are associated. INTRODUCTION Along a shoreline, the water bathing the shore is the first encountered by larvae spawned in the intertidal zone, and the last parcel of water they must cross before they can settle at the shore. Nearshore currents could retain larvae near their release site, causing high recruitment in the natal population and generating relatively closed populations. Alternately, larvae could be flushed from the waters nearshore. The larvae of many intertidal and shallow subtidal species go through development in waters over the continental shelf. At the end of their planktonic development, these larvae must migrate back to shore to settle. Flow patterns immediately adjacent to shore may prevent or aid this shoreward migration. Flow patterns in the very nearshore waters (here defined as waters 1 km from shore) may, by altering the supply of larval settlers, play an important role in regulating intertidal community structure. The waters bathing the shore are also an important source of food in the form of phytoplankton and zooplankton for intertidal and shallow subtidal filter feeders. The concentration, productivity and flux of these particles to shore affect benthic secondary productivity. Unfortunately for intertidal ecologists, the oceanography and particularly the biological oceanography of waters immediately adjacent to shore has not been well studied. Owing to a variety of engineering problems, the physical oceanography of sandy beach surf zones has been actively studied (Wright, 199). The biological doi: 1.193/plankt/fbg9, available online at Journal of Plankton Research 2(1), # Oxford University Press; all rights reserved Downloaded from on 14 June 218

2 JOURNAL OF PLANKTON RESEARCH j VOLUME 2 j NUMBER 1 j PAGES j 23 oceanography of these environments has also received some attention, particularly with regard to surf zone diatoms (McLachlan, 1983), but waters adjacent to rocky shorelines have received less attention. Researchers in Australia have studied the nearshore circulation adjacent to shore in the Great Barrier Reef and the effect of this circulation on zooplankton distributions (Hamner and Hauri, 1977, 1981; Alldredge and Hamner, 198; Wolanski et al., 1989; Willis and Oliver, 199). They have identified a number of topographically controlled circulation patterns that can concentrate zooplankton (Wolanski and Hamner, 1988). Concentrations of zooplankton orders of magnitude higher than in surrounding waters were found in convergent fronts generated by the interaction of tidal currents with shoreline topography. Fronts and associated zooplankton concentrations persisted as long as the flow regime persisted, but these systems were primarily tidally driven and, with change in the tide, topographically generated fronts vanished and zooplankton concentrations dissipated. Archambault and co-workers (Archambault et al., 1998, 1999; Archambault and Bourget, 1999) investigated the role of shoreline configuration (in or out of an embayment) and embayment size on the abundance of zooplankton, larval settlement and growth of a filter feeder (Mytilus edulis). These studies were carried out in the St Lawrence Estuary, Canada, another tidally dominated system. They found consistently higher concentrations of zooplankton within embayments than in waters outside, and attributed this to local retention and production of meroplankton. Larval settlement and the growth rate of mussels tended to be higher within embayments than outside them. They attributed these results to lower flow rates within bays. Flow rates outside embayments were consistently high enough to inhibit filtration rates, while, inside the bay, current speeds were never so high. In this, another tidally dominated system, topographically generated flow fields influenced the distribution of zooplankton. The settings that have been studied are dominated by tides and topographically generated currents fluctuated with the direction of tidal currents. Along many coasts, the dominant currents, particularly alongshore currents, are due to winds. Winds and associated wind-driven currents can persist for days, sometimes weeks. These more persistent currents may generate secondary circulation patterns (e.g. fronts, eddies, etc.) that cause relatively long-term alterations in the distribution of zooplankton, alterations that may, by affecting the supply of larvae, food particles and nutrients, play an important role in the ecology of intertidal and shallow subtidal populations. The research reported here took place along the Oregon coast. Here we have observed shore-parallel foam lines and associated changes in water coloration to persist for days at the mouth of small coves and bays. The accumulation of foam suggests that we are observing convergences, and the associated change in water coloration suggests that the convergences delineate fronts separating water masses, one in the cove or bay and the other offshore. In several geographic settings we collected physical oceanographic data across these foam lines to test the hypothesis that they are fronts separating water masses. We collected zooplankton samples to test the hypothesis that the foam lines separate zooplankton communities. In a companion paper (McCulloch and Shanks, 23) we tested the hypothesis that larval settlement varied across the foam line at Sunset Bay, Oregon. METHOD CTD data and zooplankton samples were collected along transects through Sunset Bay, Cape Arago, Oregon ( N, W; Figure 1A) during the summers of 1997 (18 and 2 July, 1 August), 1999 (2 June, 2 July and 8 September) and 2 (19 and 21 July, 16 and 24 August). In 2, we also sampled transects at Shore Acres and Miller s Cove, Cape Arago, Oregon ( N, W and N, W, respectively; Figure 1A) and Nellies Cove, Port Orford, Oregon ( N, W; Figure 1B). Physical oceanographic data were collected with a Seabird Model 19 CTD equipped with a flow-through pump. Chlorophyll (Chl) fluorescence was measured with a Wet Star in situ fluorometer on the CTD. The fluorescence measurements were not calibrated against extracted Chl samples; hence, values reported are relative Chl concentrations. Transects were oriented along the long axis of each cove sampled and perpendicular to shore at Shore Acres. Sampling began as close to shore as possible, usually <1 m, and extended offshore 1 2 km. Across foam lines and close to shore, stations were 1 2 m apart. In the ocean seaward of the foam line, station spacing increased to m. Station location and the location of the adjacent shore were determined with a GPS. Zooplankters were collected with vertical plankton tows that extended from the bottom (or 2 m depth) to thesurface.towswererepeateduntil1 2m 3 of water were filtered. During this study, two different sized 3 mm mesh nets were used, 2 and cm in diameter. A flow meterwouldnotfitinthesmallernet;volumefiltered was equal to the length through which the net was pulled times the mouth area of the net. The larger net was equipped with a flow meter. Samples were preserved with CaCO 3 -buffered formalin. Downloaded from on 14 June

3 A. L. SHANKS ET AL. j VERY NEARSHORE FRONTS AND PLANKTON DISTRIBUTION Fig. 1. Study sites around Cape Arago (A) and Port Orford (B), Oregon. Sampled transects are indicated by the long lines with arrows at both ends. In the laboratory, zooplankton, selected phytoplankton and detritus were enumerated. The phytoplankton and detritus data will be reported in a separate paper (McCulloch and Shanks, 23). Larger zooplankters were enumerated with the aid of a dissecting microscope. Samples were washed free of formalin on a 3 mm sieve, transferred to a 2 ml beaker and, with the aid of an electronic balance, water was added to make Downloaded from on 14 June

4 JOURNAL OF PLANKTON RESEARCH j VOLUME 2 j NUMBER 1 j PAGES j 23 the volume up to 2 ml (2 g). The sample was homogenized by vigorous random stirring, and a 12 ml subsample was removed with a Stempel pipette (Omori and Ikeda, 1984). Aliquots (1 ml) of the smaller zooplankton were removed following this procedure, placed in Sedgewick Rafter slides and enumerated with the aid of a compound microscope. Meroplankters were identified using identification keys in Shanks (Shanks, 21). Subsamples were counted until at least 1 individuals of the more common organisms had been enumerated. This yielded a sample standard deviation of 1% for the more abundant organisms and of between 1 and 2% for the less common species (Venrick, 1978). Owing to the limited resources that could be devoted to this project, only a portion of the net tow samples could be enumerated. Samples from three dates at Sunset Bay (1 August 1997, 2 June 1999 and 2 July 1999) and one sample set from Nellies Cove (17 August 2) were enumerated. Dates were haphazardly selected. All samples from Shore Acres and Miller s Cover were enumerated. Observations were made at several sites around Cape Arago to determine conditions under which shoreparallel foam lines formed and the amount of time they were present. Observations were made roughly every other day, starting in January 2 and continuing through September. Observations of Miller s Cove and Sunset Bay were made from an overlook just south of the entrance to Sunset Bay State Park (Figure 2A, #1). Observations of Shore Acres and Simpson Reef were made from the Simpson Reef overlook parking area (Figure 2A, #2). Observations at Cape Arago and South Cove were made just to the north of the Cape Arago overlook (Figure 2A, #3) and adjacent to the Sir Francis Drake historical monument (Figure 2A, #4), respectively. For each sample, the observer noted wind speed and direction, estimated wave height and noted the type of foam line present. Shore-parallel foam lines were defined as continuous, uninterrupted foam lines extending parallel to shore for hundreds of meters. If they were located across the mouth of a bay or cove, they needed to extend completely and continuously across the bay mouth. See, for example, the foam lines in the aerial photographs of Sunset Bay and Miller s Cove (Figure 2A) and Whale Cove, Depoe Bay and Pirates Cove (Figure 2B). Foam lines that did not extend completely across the mouth of a bay or cove (e.g. Rocky Creek Cove; Figure 2B) were not counted as shoreparallel foam lines. Along exposed shorelines (e.g. Shore Acres/Simpson Reef and Cape Arago) and during periods of larger waves, foam lines associated with rip currents were frequently observed. Rip current foam lines are generally lobed with the foam line defining the outer limit of the current (e.g. Figure 2D). To determine the spatial distribution of shore-parallel foam lines along the coast, a large collection of aerial survey photographs at the National Oceanographic and Atmospheric Administration (NOAA) website ( was inspected. All coastal photographs between the Straits of Juan de Fuca, Washington, and San Francisco Bay, California, posted (as of August 21) at this NOAA website were inspected for shore-parallel foam lines associated with headlands and small bays and coves. Figure 2 presents examples of these photographs. Categorization of foam lines in the photographs was the same as that used in the observations made at Cape Arago (described above). In addition, we noted sea state (calm versus rough) and the direction from which the seas were coming, from which we inferred wind direction. RESULTS Sunset Bay Site description A crescent-shaped beach and stream are at the head of Sunset Bay, a small bay on the southern Oregon coast (Figures1Aand2A).About3mfromshore,thebay becomes tightly constricted, then opens up to form the outer portion of the bay. Away from the beach, the shoreline is rocky; two reefs form the mouth of the bay. Depending on thetide,thebaymouthis.7.8kmfromthebeach. A shore-parallel foam line or, frequently, a pair of parallel foam lines extends from the rock reef on the north side of the bay to the reef to the south (Figure 1A). The foam line is generally connected 2 m landward of the most seaward rocks. Repeated observations of the foam line over many days (see below) and during field sampling indicated that the foam line does not change position with the stage of the tide. The shape, position and continuity of the foam line, however, changed with wind direction and sea state (data presented below). At times, we observed a clear change in water coloration across the foam line. Dye released to either side of the foam line moved steadily toward the foam, and then was downwelled beneath the foam. The foam line invariably contained large amounts of floating detritus. The foam line(s) at the mouth of Sunset Bay appears to delineate a convergence zone(s) associated with a front that separates the waters in the bay from those in the coastal ocean. The CTD sampling was designed to test this hypothesis. Physical oceanography Throughout the three summers of sampling, salinity varied little along the transects; the total variation in all cases was Downloaded from on 14 June

5 A. L. SHANKS ET AL. j VERY NEARSHORE FRONTS AND PLANKTON DISTRIBUTION Fig. 2. Examples of foam lines and rip currents in aerial photographs from the NOAA Mapfinder website. (A) Cape Arago, Oregon. Oceanographic sampling presented in this paper took place at Miller s Cove, Sunset Bay and Shore Acres. Numbers in white circles indicate locations from which 9 months of observations were made of the presence or absence of foam lines around Cape Arago. (B) Coastline near Depoe Bay, Oregon. Note foam lines extending across the mouths of Pirate Cove and Whale Cove, and the incomplete foam line across the mouth of Rocky Creek Cove. (C) North of Depoe Bay, Oregon. Note foam lines across mouths of Boiler Bay and Pirate Cove. (D) Foam line generated by a rip current. <.. The lowest salinity was often adjacent to the beach, undoubtedly due to Big Creek, which empties into Sunset Bay. In summer, freshwater input is small (.8 m 3 s 1 in June and <.2 m 3 s 1 in July, August and September; Coos County Water Resources Department) and has little effect on salinity in the Bay. In winter, however, larger freshwater input (at times > m 3 s 1 ) may cause Sunset Bay to behave as a small estuary. Ten CTD transects were made across Sunset Bay during three summers. On nine cruises, winds were Downloaded from on 14 June

6 Depth, m JOURNAL OF PLANKTON RESEARCH j VOLUME 2 j NUMBER 1 j PAGES j July Sept July July Aug June July July Aug Aug Distance Offshore, Km Fig. 3. Temperature ( C) isotherms from CTD transects sampled at Sunset Bay, Oregon. Diamonds above each figure indicate station locations. The black area at the bottom left of each figure represents the bottom. Downward arrows indicate the location of the foam line and front at the mouth of Sunset Bay. upwelling favorable (i.e. NW) and on one (1 August 1997) winds were downwelling favorable (i.e. SW). During upwelling winds, a foam line extended completely across the bay mouth. We observed a shallow (< m deep) lens of warmer water landward of the foam line and in Sunset Bay with water.2. C cooler seaward of the foam line (Figure 3). Warmest waters were closest to the beach. On 1 August 1997, during downwelling winds, the foam line did not extend across the mouth and there was no difference in surface water temperatures. During upwelling winds, the deeper isotherms (> m) were bent downward just seaward of the bay mouth and Downloaded from on 14 June

7 Depth, m A. L. SHANKS ET AL. j VERY NEARSHORE FRONTS AND PLANKTON DISTRIBUTION foam line (Figure 3). In addition, on several days (2 June 1999, 8 September 1999, 19 and 21 July 2, 16 and 24 August 2; Figure 3), isotherms appeared to dome upward at or just seaward of the foam line. On 1 August 1997, the deeper isotherms were bent upward and were roughly parallel to the bottom. On three days (8 September 1999, 19 July 2 and 24 August 2), the Chl concentration was low (<1 mgl 1 ) and varied little along the transects (Figure 4). On days with both higher Chl concentrations and upwelling conditions, the Chl concentration was higher seaward of the foam line and bay mouth than landward (Figure 4). On 2 days with upwelling-favorable winds (21 July and 16 August 2), the highest concentrations of Chl were distributed vertically through the water column just seaward of the foam line (Figure 4). On 1 August 1997, July Sept July July Aug July June Aug July Aug Distance Offshore, Km Fig. 4. Plots of Chl (mg l 1 ) from CTD transects sampled at Sunset Bay, Oregon. Diamonds above each figure indicate station locations. The black area at the bottom left of each figure represents the bottom. Downloaded from on 14 June

8 JOURNAL OF PLANKTON RESEARCH j VOLUME 2 j NUMBER 1 j PAGES j 23 during downwelling winds, the concentration of Chl was highest inside the bay. Seaward of the bay mouth, Chl concentrations were around 6 mg l 1, while inside the bay, adjacent to the beach, the concentration was up to 2 mg l 1 (Figure 4). During downwelling winds, surface waters within the bay may not be separated from the coastal waters, and high concentrations of Chl can be found in the bay. In contrast, during upwelling conditions, the foam line at the bay mouth delineated a convergent front separating warmer waters, generally characterized by low Chl concentrations, within the bay from cooler offshore waters with higher Chl concentrations and deeper isotherms tilted downward seaward of this front. Zooplankton distribution Zooplankton data are presented for 1 August 1997, 2 June 1999 and 2 July On the first two dates, three replicate samples were collected at two stations landward of the front and one seaward. On the third date, samples were collected at each station along the CTD transect. Concentrations of zooplankton landward of the foam line were compared with seaward concentrations using a Kruskal Wallis one-way ANOVA by ranks. With the 2 July 1999 data, stations landward and seaward of the foam line were considered replicate samples from their respective habitats. To assist in the description of the distribution of zooplankton taxa, we sorted organisms into groups using cluster analysis. Zooplankters were separated into groups based on their spatial distribution. The concentrations of zooplankton were standardized such that the mean equaled zero and the standard deviation equaled one and, using the Wards Method, zooplankton were grouped into clusters by Euclidean distance (StatSoft, 1994). On 2 June and 2 July 1999, winds were upwelling favorable. On both dates, there were significantly higher concentrations of nauplii and adult calanoid copepods seaward of the front than landward (Figure ; Table I). Seaward of the foam line, concentrations of nauplii and adult calanoid copepods were of the order of 1 4 m 3 (Figures and 6). Landward concentrations fell to <1 3 m 3. Several types of meroplankters had similar distributions. On June , barnacle nauplii stage 3, barnacle nauplii stages 4, and 6 combined, polychaete larvae with more than four setigers, mussel larvae, total decapod zoeae and echinoplutei had distributions similar to those of the calanoid copepods; concentrations of these meroplankton decreased landward of the foam line by almost an order of magnitude (Figure ). Cluster analysis grouped these taxa with calanoid copepods (Figure 7). On 2 July 1999, barnacle nauplii stages 4, and 6 combined, polychaete larvae with more than three setigers, mussel larvae and total decapod zoeae exhibited distributions similar to those of the calanoid copepods (Figure ). In most cases, statistical analysis indicated that significantly more larvae were present seaward of the front; concentrations fell by around an order of magnitude landward of the front (Figure ; Table I). Cluster analysis grouped these meroplankters with calanoid copepods and their nauplii (Figure 7). A set of organisms tended to be more concentrated landward of the front. On 2 June 1999, harpacticoid copepods, barnacle cyprids, barnacle nauplii stages 1 and 2, and Littorina egg cases were grouped in the same cluster (Figure 7). However, only Littorina egg cases were significantly more concentrated landward of the front (Figure 6). Barnacle cyprids and harpacticoid copepods were more abundant landward of the front, but not significantly (Figure 6), and there was little variation in the concentration of barnacle nauplii stages 1 and 2. On 2 July 1999, the nearshore cluster was composed of harpacticoid copepods, polychaete larvae with three or fewer setigers, barnacle nauplii stages 1 and 2, barnacle nauplii stage 3, barnacle cyprids, Littorina egg cases and veligers, and phoronid larvae (Figure 7). Harpacticoid copepods, polychaete larvae with three or fewer setigers, barnacle nauplii stages 1 and 2, and Littorina egg cases and veligers were at significantly higher concentrations landward of the front (Table I). Low concentrations of barnacle nauplii stage 3, barnacle cyprids and phoronid larvae were found seaward of the front and close to shore; highest concentrations were at the stations.4 and.7 km from shore (Figure ). On these 2 days with upwelling winds, the Sunset Bay front separated two zooplankton communities: one community was in the bay landward of the front; the other was seaward of the front. Differences in concentration across the front between stations separated by only a few hundred meters or less were, in many cases, as large as an order of magnitude. On 1 August 1997, winds were downwelling favorable and zooplankton distributions were quite different than on days with upwelling winds. Calanoid copepods and their nauplii were far less abundant, and their concentration did not change significantly across the bay mouth (Figure 8); highest concentrations were actually found inside the bay. There were no significant differences in the concentrations of gastropod veligers, echinoplutei, total decapod zoeae, mussel larvae, barnacle nauplii stages 4, and 6, starfish brachiolaria and polychaete larvae with more than three setigers across the bay mouth; the highest concentrations were, as with copepods, usually found at the station just inside the bay mouth (Figure 8). No significant differences were found in the distributions of Littorina egg cases or veligers (Figure 8); there was little variation in their concentrations Downloaded from on 14 June

9 A. L. SHANKS ET AL. j VERY NEARSHORE FRONTS AND PLANKTON DISTRIBUTION 2 1 A 26.1 B 2 8 Nauplii 1,2 Calanoid Adults 6 Nauplii 3 1 Nauplii Nauplii 4, C Harpacticoids Zoeae D Cyprids Mussels E Polychaete Larvae <4 Setigers F Littorina Eggs 26.1 >4 Setigers 2.6 Veligers G Phoronid Larvae Distance Offshore, km Fig.. Distribution of zooplankton on 2 July 1999 during upwelling winds in relation to the foam line and front at the mouth of Sunset Bay, Oregon. Density is plotted as the fine dotted line and the vertical dashed lines indicate the location of the foam line. 2.1 across the bay mouth. As on the upwelling sample dates, no significant variations in the distributions of barnacle nauplii stages 1 and 2, and barnacle cyprids were observed; highest concentrations were found at the station just inside the bay mouth (Figure 8). Significant variation was only found in the distribution of harpacticoid copepods (Figure 8); significantly higher concentrations were caught at stations inside the bay. Cluster analysis produced two groups (Figure 7): one composed of organisms that had their highest concentrations at the station just inside Sunset Bay; the second composed of organisms with either highest concentrations at the most seaward station or little variation in concentration across the transect. On this date, with downwelling winds and oceanographic conditions indicative of a relaxation event, we found little variation in zooplankton distributions across the mouth of Sunset Bay. Miller s Cove Site description Miller s Cove is located just to the north of Sunset Bay (Figure 1). The mouth of the cove has roughly the same orientation as Sunset Bay. The north side of Miller s Cove is composed of Gregory Point (an island). Situated within the mouth of Miller s Cove are two narrow islands oriented parallel to Gregory Point. Deep channels separate the islands. At the landward end of Miller s Cove is a shallow opening (nearly closed at low tide) that separates Gregory Point from the mainland and Downloaded from on 14 June

10 JOURNAL OF PLANKTON RESEARCH j VOLUME 2 j NUMBER 1 j PAGES j 23 Table I: Results of Kruskal Wallis one-way ANOVA by ranks comparing stations sampled on 2 July 1999 at Sunset Bay that were seaward ( 1 km from shore, n = 4) and landward of the foam line (< 1 km from shore, n = 4) Organism Distribution a H P Calanoid copepods Copepod nauplii Harpacticoid copepods Barnacle nauplii stages 1, Barnacle nauplii stage 3. 1 Barnacle nauplii stages 4,, Barnacle cyprids Polychaete larvae with 3 setigers Polychaete larvae with >3 setigers _ Mussel larvae Littorina egg cases Littorina veligers Phoronid larvae Total zoeae Data are plotted in Figure 7. a Larvae that were significantly more abundant landward and seaward of the front and foam line are indicated by and + signs, respectively. connects Miller s Cove to Lighthouse Beach. Two foam lines were often present across the mouth of Miller s Cove. The foam lines extended from the landward end of the two islands in the cove, south to the rock reef separating Miller s Cove from Sunset Bay (Figure 2). We have one complete data set of physical and biological data collected in Miller s Cove on 24 August 2. These data were collected during upwelling-favorable winds. Physical oceanography Surface temperature fronts associated with foam lines were observed at.2 and. km from shore (Figure 9). Surface water landward of the foam lines was. C warmer than seaward. Like Sunset Bay, the warmest waters were found within a couple of hundred meters of shore (Figure 9). Seaward of the foam lines, at.6 km from shore, subsurface isotherms were domed upward, and landward of this dome they bent steeply downward so that the thermocline (centered around the 9. C isotherm) contacted the bottom at.1 km from shore. Surface Chl concentrations were higher seaward of the foam line than landward, with lowest values found immediately adjacent to shore (Figure 1). High concentrations of Chl were found just below the thermocline where it was bent downward toward the bottom. The highest concentrations were found beyond.8 km from shore and below 1 m depth. The oceanographic data, though limited, suggest that, like Sunset Bay, foam lines at Miller s Cove delineate fronts separating a warmer Chl-poor water mass from a cooler offshore water mass with higher Chl concentrations. Zooplankton distribution Zooplankton samples were collected at each CTD station. The cluster analysis suggested that there were two patterns of distribution (Figure 11): organisms with highest concentrations seaward of the inner foam line and front (i.e. seaward of.2 km); and those with their highest concentrations landward of the inner foam line (i.e. landward of.2 km). The former group of organisms, adult and nauplii calanoid copepods, barnacle nauplii stages 4, and 6, zoeae, polychaete larvae with three or fewer setigers and polychaete larvae with more than three setigers, were significantly more concentrated seaward of the front (Table II). Barnacle cyprids, phoronid larvae, gastropod veligers, mussel larvae, and planula, were found at higher, but not significantly higher, concentrations seaward of this front (Figure 1; Table II). In contrast, harpacticoid copepods and Littorina egg cases were most abundant at the stations landward of the inner foam line. However, only Littorina egg cases were significantly more concentrated landward of this front (Table II). These data suggest that associated with the foam lines in Miller s Cove are abrupt changes in the community structure and abundance of a variety of zooplankters. Downloaded from on 14 June

11 A. L. SHANKS ET AL. j VERY NEARSHORE FRONTS AND PLANKTON DISTRIBUTION Copepod Nauplii Calanoid Copepods 4 3 o Stages 1,2 Stage 3 Stages 4,,6 Barnacle Nauplii < 3 Setigers > 4 Setigers Polychaete Larvae Cyprids Mussel Larvae 1 3 Landward of Front Near Beach 1 2 Landward of Front 1 Seaward of Front Littorina Egg Cases Decapod Zoea Harpacticoid Copepods Pseudo-Nitzschia spp. Nocticluca scintillans Terrestrial Detritus Fig. 6. Distribution of zooplankton on 2 June 1999 during upwelling winds in relation to the foam line and front at the mouth of Sunset Bay, Oregon. Plotted are the mean and standard error of three replicate tows. Shore Acres Site description The Shore Acres transect was located. km south of Sunset Bay and 3 m north of the Shore Acres State Park wave viewing kiosk (Figure 1). The shoreline from Sunset Bay south for 3 km is oriented roughly toward the NW and composed of rocky reefs and cliffs (Figure 2). Transects were sampled on 19 July and 16 August 2, during upwelling winds. The transect on 19 July was only Downloaded from on 14 June

12 JOURNAL OF PLANKTON RESEARCH j VOLUME 2 j NUMBER 1 j PAGES j 23 Fig. 7. Cluster analysis of the distribution of zooplankton at Sunset Bay, Oregon, on 2 June and 2 July 1999 during upwelling winds, and on 1 August 1997 during downwelling winds. Downloaded from on 14 June

13 Number Per Meter Cubed A. L. SHANKS ET AL. j VERY NEARSHORE FRONTS AND PLANKTON DISTRIBUTION 7 H=.7 p=.8 H=.7 p=.8 Landward of Front Near Beach Landward of Front 2 Seaward of Front Copepod Nauplii Calanoid Copepods 1 H=.4 p=.2 H=.6 p=.44 H=.3 p=.61 H=.1 p=.8 H=3.3. p=.7 1 Harpacticoid Copepods Gastropod Veligers Echinoplutei Total Crab Zoeae Mussel Larvae 2 H=. p=1 H=2.4 p=.12 H=2.4 p=.12 H=8.3 p= Stages 1,2 Stage 3 Stage 4,,6 Barnacle Nauplii Barnacle Cyprids 8 H=2. p=.11 H=.8 p=.36 H=.4 p=.2 H=.6 p= Littorina Egg Cases Littorina Veligers Brachiolaria Polychaete Larvae >3 Setigers Fig. 8. Distribution of zooplankton on 1 August 1997 during downwelling winds in relation to the foam line and front at the mouth of Sunset Bay, Oregon. Plotted are the mean and standard error of three replicate tows. Values above graphs are results of Kruskal Wallis one-way ANOVA by ranks. 4 m long, while that on 16 August was 1.4 km long. On both dates, data were also collected at Sunset Bay. Physical oceanography On both days, the distribution of temperature was similar. Approaching the shore, the thermocline isotherms tilted upward toward the surface. On 19 July, this occurred within 1 m of shore, and the thermocline did not contact the surface (Figure 12). A foam line was located 2 m from shore. On 16 August, the upward tilt was stronger and began.6 km from shore. A thermal front and associated foam line were located.4 km from shore, where the thermocline (9.9 C isotherm) contacted the surface. Landward of this front, surface waters were cooler and denser than on the seaward side, and isotherms cooler than 9.8 C were bent downward (Figures 12 and 13). The lowest concentrations of Chl were found in the waters within several hundred meters of shore and extended throughout the water column (Figure 12). Higher Chl concentrations were within or below the thermocline, and just seaward of these waters with the lowest Chl Downloaded from on 14 June

14 Depth, m JOURNAL OF PLANKTON RESEARCH j VOLUME 2 j NUMBER 1 j PAGES j Temperature Chl a Distance Offshore, Km Fig. 9. Plots of temperature ( C) and Chl (mg l 1 ) from CTD transects at Miller s Cove, Oregon, on 24 August 2 during upwelling winds. Diamonds and downward arrows above each figure indicate station and foam line locations, respectively. The black area at the bottom left of each figure represents the bottom A Calanoid Adult Nauplii Harpacticoids Zoeae C.. 1. Polychaete Larvae <3 >3 Setigers Larval Gastropods Phoronids B G Distance Offshore, km E Barnacle Nauplii 1,2 3 4,,6 Cyprids Mussels Littorina Eggs D F Fig. 1. Cluster analysis of the distribution of zooplankton at Miller s Cove, Oregon, on 24 August 2. concentrations. On 16 August, the highest Chl concentrations were beneath the front where the thermocline intercepted the surface (Figure 12). Distributions of oceanographic parameters at Sunset Bay on 19 July and 16 August (Figures 3 and 4) were quite different from those observed at Shore Acres (Figure 12). At Sunset Bay on 19 July, there was a lens of warm water adjacent to shore, and at the bay mouth isotherms were bent downward. No lens of warm water was observed off Shore Acres and isotherms tilted upward as shore was approached (Figure 12). At Sunset Bay on 16 August, there was also a lens of warm water adjacent to shore and isotherms within the thermocline ( C) were horizontal. At Shore Acres, waters Downloaded from on 14 June

15 A. L. SHANKS ET AL. j VERY NEARSHORE FRONTS AND PLANKTON DISTRIBUTION Copepod Nauplii Barnacle Nauplii 1,2 Gastropod Larvae Calanoid Copepods Total Zoeae Barnacle Nauplii 4,,6 Barnacle Nauplii 3 Mussel Larvae Cyprids Polychaetes-Early Polychaetes-Late Phoronid Larvae Harpacticoid Copepds Littorina Egg Cases Cluster #1 - Highest Concentrations Found Seaward of the Inner Front Cluster #2 - Highest Concentrations Found Landward of the Inner Front Fig. 11. Distribution of zooplankton on 24 August 2 during upwelling winds in relation to the foam line and front at the mouth of Miller s Cove, Oregon. Table II: Results of Kruskal Wallis one-way ANOVA by ranks comparing stations sampled on 24 August 2 at Miller s Cove that were seaward (.2 km from shore, n = ) and landward of the foam line (<.2 km from shore, n = 3) Organism Distribution a H P Calanoid copepods Copepod nauplii Harpacticoid copepods Barnacle nauplii stages 1, Barnacle nauplii stage Barnacle nauplii stages 4,, Barnacle cyprids Polychaete larvae with 3 setigers Polychaete larvae with >3 setigers +..2 Mussel larvae Littorina egg cases Phoronid larvae Total zoeae Data are plotted in Figure 12. a Larvae that were significantly more abundant landward and seaward of the front and foam line are indicated by and + signs, respectively. close to shore were cooler than offshore and the thermocline was tilted upward, contacting the surface. Although Sunset Bay and Shore Acres were sampled only an hour apart and are separated by < m, the distribution of water masses was dramatically different. Foam lines and fronts were present at both locations, but very different distributions of oceanographic properties at the sites suggest that different mechanisms must be generating the foam lines and fronts. Zooplankton distribution The zooplankton data from 19 July and 16 August 2 are presented in Figures 13 and 14. Data from the longer transect sampled on 17 August were analyzed statistically and this analysis suggests there were two general patterns of zooplankton distribution (Figure 1). The first pattern was typified by the distribution of calanoid copepods and their nauplii. Concentrations were relatively high at the most seaward stations, peaked at the station on the seaward side of the thermal front (Figure 13A,.7 km), and then fell to low concentrations at stations landward of the thermal front, with lowest concentrations at the station adjacent to shore. Note, however, that concentrations varied by only about a factor of four over the entire transect. Organisms with similar distributions to calanoid copepods included gastropod veligers, mussel larvae, nudibranch veligers, zoeae and Littorina veligers (Figure 16). Only zoeae were Downloaded from on 14 June

16 Depth, m JOURNAL OF PLANKTON RESEARCH j VOLUME 2 j NUMBER 1 j PAGES j July 16 August Temp. Temperature Chl a Chl a Distance Offshore, Km Fig. 12. Plots of temperature ( C) and Chl (mg l 1 ) from CTD transects at Shore Acres, Oregon, on 19 July and 16 August 2, days with upwelling winds. Diamonds and downward arrows above each figure indicate station and foam line locations, respectively. The black area at the bottom left of each figure represents the bottom. significantly more concentrated seaward of the front (Table III). The second pattern of zooplankton distribution can be typified by the distribution of polychaete larvae with three or fewer setigers (Figure 13E). Seaward of the thermal front, concentrations were very low, increased rapidly landward of the front and then fell sharply (by a factor of 1) within 2 m of shore. Organisms with similar distributions included harpacticoid copepods, all stages of barnacle nauplii, barnacle cyprids, polychaete larvae with more than three setigers and planula. Of this list, however, only barnacle stages 1 and 2, and polychaete larvae with three or fewer setigers were significantly more concentrated landward of the front (Table III). Distributions of zooplankton along the shorter transect sampled on 19 July 2 appear similar to those within m of shore on 16 August (Figures 13 and 1). On both dates, relatively high concentrations of copepods and copepod nauplii were found within 1 m of shore. As on 16 August, relatively high concentrations of all stages of barnacle nauplii, bivalve larvae, gastropod veligers and polychaete larvae were found within 2 m of shore. As on 16 August, cyprids and harpacticoids were most abundant close to shore. The difference in zooplankton distributions at Sunset Bay (Figures and 7) and Shore Acres (Figures 13 and 14) are most clearly demonstrated with the distribution of calanoid copepods. At Sunset Bay, calanoid copepods were significantly more abundant seaward of the front, and their numbers fell by at least an order of magnitude across the front. In contrast, at Shore Acres, while the highest concentration of calanoid copepods was on the seaward side of the front, numbers remained high to within 2 m of shore, and even at the most landward station their numbers were reduced by only a little more than half. Unlike at Sunset Bay, offshore zooplankton taxa were found at relatively high concentrations within 2 m of shore at Shore Acres. Nellies Cove Site description Nellies Cove is located. km from the breakwater at Port Orford, Oregon (Figure 1) and 12 km south of Cape Blanco, a major upwelling center. At Port Orford, a headland (altitude 1 m) juts out into the ocean, forming a bight to the south that is sheltered from the north winds (Figure 1). Nellies Cove is composed of two narrow (<1 m wide) coves surrounded by high cliffs (Figure 1). Transects began <3 m from the cobble beach at the head of the western arm of Nellies Cove and ran along the long axis of the cove, and out to sea. Physical oceanography On 17 and 31 August 2, a slender foam line (width 1 m) extended across the mouth of Nellies Cove. The foam line did not extend straight across the mouth, but rather bulged out from the bay mouth m. On both dates, we observed a change in water Downloaded from on 14 June

17 A. L. SHANKS ET AL. j VERY NEARSHORE FRONTS AND PLANKTON DISTRIBUTION 8 A 26.3 B Barnacle Nauplii 1,2 3 4,, Calanoid Nauplii Adults C Harpacticoids Zoeae D Cyprids Mussels E Polychaete Larvae 1 <3 Setigers >3 Setigers F Zoeae 26. Nudibranch Larvae Distance Offshore, km Fig. 13. Distribution of zooplankton on 16 August 2 during upwelling winds in relation to the foam line and front at Shore Acres, Oregon. Density is plotted as the fine dotted line and vertical dashed lines indicate the location of the foam line. coloration associated with this foam line: bay waters were milky in appearance, while those offshore were clear. Warmest waters were located inside Nellies Cove and landward of the foam line (Figure 16). The fluorometer failed on 17 August. On 31 August, Chl concentrations were highest below the foam line and front at the cove mouth (Figure 16); concentrations were up to seven times higher than to either side of the front. The foam line delineated a front in water coloration and temperature. At. (17 August) and 1.1 km (31 August) from shore we observed a second surface thermal front (Figure 16). At 1 1. km from shore, seaward of the shelter of the Port Orford Bight, the mixed layer deepened and isotherms below 1 or 2 m tilted downward (Figure 16). Zooplankton distribution On 17 August 2, there were two patterns of zooplankton distribution (Figure 17). Most organisms were more abundant seaward of the foam line at the cove mouth (Figure 18). For example, calanoid copepods were at concentrations around 1 4 m 3 seaward of the foam line, but concentrations fell to 1 2 m 3 inside the cove. Significantly higher concentrations of copepod nauplii, calanoid copepods, barnacle nauplii stage 3, barnacle nauplii stages 4, and 6, phoronid larvae, toredo larvae and total zoeae were found seaward of the front (Table IV). The cluster analysis identified a second group of organisms most concentrated around the foam line at the cove mouth (Figure 17). The distributions of harpacticoid copepods and their nauplii were particularly striking (Figure 18). At the two stations centered on the foam line, harpacticoid nauplii were at concentrations between 1 4 and nearly 1 m 3, but just tens of meters away, at adjacent stations, their numbers fell to near zero. Downloaded from on 14 June

18 JOURNAL OF PLANKTON RESEARCH j VOLUME 2 j NUMBER 1 j PAGES j 23 Fig. 14. Distribution of zooplankton on 19 July 2 during upwelling winds in relation to the foam line and front at Shore Acres, Oregon. Density is plotted as the fine dotted line and vertical dashed lines indicate the location of the foam line. Copepod Nauplii Hermit Crab Zoeae Gastropod Veligers Calanoid Copepods Mussel Larvae Nudibranch Veligers Harpacticoid Copepds Barnacle Nauplii 1,2 Cyprids Barnacle Nauplii 3 Barnacle Nauplii 4,,6 Polychaetes-Late Polychaetes-Early Cluster #1 - Highest Concentrations Found Seaward of the Front Cluster #2 - Highest Concentrations Found Landward of the Front Fig. 1. Cluster analysis of the distribution of zooplankton at Shore Acres, Oregon, on 16 August 2. Conditions favoring shore-parallel foam lines During periods of large waves (nearshore amplitude >2 m), shore-parallel foam lines were seldom present (Table V). None was present at Sunset Bay, Miller s Cove, Cape Arago or South Cove during these conditions. At Simpson Reef/Shore Acres, out of 21 largewave days, there were shore-parallel foam lines on four. On days with large waves, if foam lines were present, they were nearly always lobed shaped (Figure 2D), indicative of foam lines generated by rip currents. Shoreparallel foam lines were much more common when waves were smaller (nearshore amplitude <2 m; Table V). On several winter days we observed Sunset Bay during large wave events. Larger waves arrived in sets, causing the sea level to rise.3 m. With diminishing Downloaded from on 14 June

19 A. L. SHANKS ET AL. j VERY NEARSHORE FRONTS AND PLANKTON DISTRIBUTION Fig. 16. Plots of temperature ( C) and Chl (mg l 1 ) from CTD transects at Nellies Cove, Port Orford, Oregon, on 17 and 31 August 2 during upwelling winds. Chlorophyll data for 17 August were lost due to mechanical problems. Diamonds and downward arrows above each figure indicate station and foam line locations, respectively. The black area at the bottom left of each figure represents the bottom. wave size, excess water pushed into Sunset Bay flowed out as a narrow fast current that generated a rip current foam line extending hundreds of meters seaward from the bay mouth. When waves were large, water was rapidly flushed from the bay and exchanged with coastal water. The mouths of Sunset Bay and Miller s Cove have roughly the same north south orientation and shoreparallel foam lines were generated by the same conditions. At Sunset Bay, when waves were relatively small and winds were from the NE, SE or SW (1 days), a shore-parallel foam line was observed once (Table V). Under the same conditions at Miller s Cove (3 days), there were 4 days with shore-parallel foam lines (Table V). When waves were small and winds were other than upwelling favorable, either no foam lines were present at the bay mouths or foam lines did not extend completely across the mouth of the bays but were attached to rocks on the south side of the bays and curved sharply into the bays. When waves were small and winds were from the NW (upwelling favorable), shore-parallel foam lines extending completely across the mouths of the bays were common: 46 out of 46 days at Sunset Bay and 18 out of 21 days at Miller s Cove (Table V). South Cove opens to the south (Figure 2); shoreparallel foam lines were seldom observed. Out of 16 days with large waves, shore-parallel foam lines were present only once (Table V). Even when waves were small, shore-parallel foam lines were not common; during small waves and winds from the NE, SE or SW (28 days), shore-parallel foam lines were present on three dates. In contrast to Sunset Bay and Miller s Cove, out of 16 days with small waves and NW winds, shore-parallel foam lines were never present. The observations made at Simpson Reef overlook and Cape Arago were of more exposed sections of shoreline (Figure 2A). Both shorelines have a roughly north south orientation. At Simpson Reef overlook, the observations were made to the north of Simpson Reef itself, toward Shore Acres State Park. This is exposed coast with no offshore rocks and deeper water (>1 m depth) within tens of meters of shore. At Cape Arago, the shore is equally Downloaded from on 14 June