Response of larval fish assemblages to a riverine plume in coastal waters of the central Great Barrier Reef lagoon

Transcription

1 NOTES Lmno. Oceanogr., 4(I), 1995, 177-I , by the American Society of Limnoogy and Oceanography, Inc. Response of arva fish assembages to a riverine pume in coasta waters of the centra Great Barrier Reef agoon Abstract - Ichthyopankton assembages were compared among stations in the vicinity of a riverine pume in coasta waters of the centra Great Barrier Reef agoon, Austraia, during January Athough the pume dramaticay infuenced both community structure and abundance of arva fish, effects were temporay dynamic and taxonspecific. Muids were the ony famiy to show significanty higher densities at the pume front than either inside or outside the pume. Sampes from the pume front and coasta waters outside the pume showed simiar taxonomic affinities. These affinities appeared to be driven by the offshore movement of the pume transocating coasta arvae offshore, which aso ed to accumuation of these arvae at the pume front. Given the arge area that the pume covered and the high zoopankton biomass and secondary production in these waters, we suggest that the pume may have affected arva fish surviva and recruitment more than the associated pume front. Freshwater discharge into coasta waters is usuay manifested as riverine pumes. These pumes are shaow (typicay <5 m), buoyant enses of ow-sainity water overaying denser, more saine water (Bowman 1988). Riverine pumes are highy stratified and bioogicay active water masses and therefore have considerabe impications for the surviva and recruitment of arva fishes. Pumes and associated fronta structures present a rich food environment for arva and juvenie fish (Dagg and Whitedge 199 1). Increased growth rates in these water masses may ead to short stage durations, a concomitant decrease in predation rates, and hence high survivorship (Grimes and Finucane 199 1). The reativey high (and constant) discharge rates from arge river systems such as the Mississippi and Coumbia generate pumes with a high degree of tempora persistence (Dagg et a. 1987; Bowman 1988). Not coincidentay, these systems have aso been the focus of intensive research efforts on riverine pumes (references cited above). In coasta waters aong the east coast of Austraia, however, pumes are transient features usuay associated with cyconic rains (Woanski and Jones 198 1). The inherent unpredictabiity of pume formation in this area has meant that itte is known of the infuence of such events on panktonic assembages. The presence of Cycone Joy in January brought the city of Townsvie its wettest January on record. The timing coincided with an on-going study of the bioogica oceanography of coasta waters in the centra Great Bar- rier Reef agoon (McKinnon and Thorrod 1993). We reported extremey high zoopankton biomass and secondary production within a arge riverine pume caused by the freshwater runoff. Our objectives in the present study were to determine ichthyopankton assembages within adjacent pume and coasta water masses and at the associated front and reate the observed patterns to physica and bioogica processes acting at this time. A samping was conducted in the centra Great Barrier Reef (hereafter GBR) agoon. The ow-sainity pume and associated pume front were observed from 19 to 25 January aong the transect samped by McKinnon and Thorrod (1993, figure 1). The front was ocated visuay by a distinct coor change between turbid pume water and cearer, coasta water. Osciatoria, fotsam, and foam were aso concentrated at the fronta boundary, which aided in identifying the feature. Physica oceanographic data (sainity, temperature), chorophy a (< 1 and > 1 pm), zoopankton abundance, and egg production rates of a common coasta copepod (Acrocaanus gibber) were aso measured on two occasions during the ichthyopankton samping period. These data are presented esewhere (McKinnon and Thorrod 1993). Ichthyopankton were coected with a 75cm ring net fitted with 55-pm mesh, a standard 3-point bride system, and a caibrated Genera Oceanics digita fowmeter. The net was towed between.5 and 1 m beow the surface for 1 min at - 1 m s-. Three repicate tows were made at each of three stations on five samping occasions [on, and parae to, the pume front, and at stations 3 km inside (i.e. in pume water) and outside (i.e. coasta water) of the front]. Because we had to ocate the front visuay, a samping was conducted during dayight. Fish arvae were removed from the sampes under a dissecting microscope and identified to famiy eve. These imits to taxonomic resoution are unfortunate but unavoidabe as 177 arva identifications beow this eve in the tropica Indo- West Pacific are probematic. Grouping at the famiy eve wi, however, be a conservative measure of community differences. The one exception was the famiy Carangidae; the tribe Carangini (type A carangids) was separated from a other carangids (type B carangids). Fu taxonomic detais, aong with numbers coected, are given esewhere (Thorrod 1993). We coected 4,72 fish arvae, representing some 49 famiies, from 45 pankton tows. This data matrix, con- sisting of 45 sampes by 5 taxa, was initiay subjected to mutivariate pattern anaysis. A Bray-Curtis dissimi-

2 178 Notes Inside pume a Pume front. Outside pume. 19Jan 2 Jan 21 Jan A 22 Jan. 25 Jan. I B C. a 7 D. % a = z.- E.- I c.6 Fig. 1. Summary tabe of dendrogram from UPGMA custer anaysis (@ = -.1). Bubbe size is scaed so that the area of a bubbe is proportiona to the number of sampes from each of the spatia and tempora categories in the four custer groupings. Arrows indicate direction that bubbes sum to 1%. arity matrix was generated from the famiy-sampe data set; this matrix was then custered by means of a fexibe unweighted (UPGMA) custering strategy. The custer anaysis found that four usefu groupings can be discerned from the data (Fig. 1). The most distinct group, custer A, contained pume sampes from the first 2 d of samping. Pume sampes from the fina three cruises were ocated in custer B, aong with a sma number of sampes from both fronta and coasta water masses. There was itte evidence of a distinctive assembage associated with the pume front. Pume front sampes were grouped in both custers C and D, aong with sampes from outside the pume front. Tempora effects were aso important, with custers A and D primariy from the first 2 d of samping, whie custers B and C contained sampes from the fina 3 d. Mante (1967) and partia Mante (Smouse et a. 1986) tests were used to more rigorousy assess spatio-tempora variabiity in ichthyopankton distributions. The Bray- Curtis dissimiarity matrix formed the data matrix for Mante and partia Mante tests, with each aternative hypothesis (no effect of position reative to the pume front and no effect of cruise) cast into a mode matrix. A test of matrix correspondence (r) was cacuated between the data and mode matrices. We then used an empirica nu distribution derived from randomy permuting one of the matrices 5 times to compute the probabiity of obtaining a vaue of r at east as extreme as the observed r by chance aone..6 Significant effects of both station and samping date, aong with a significant station x date interaction, were apparent (Tabe 1). The dissimiarity matrix did, however, show more affinity to the spatia mode (r =.326, partia r =.334) than the tempora one (r =.1, partia r =.128). The nature of the spatio-tempora interactions can be seen in pots of tota fish and the five most abundant famiies through time (Fig. 2). Tota fish were initiay high within the pume, refecting the arge numbers of type A carangids captured during this time. Numbers were high at the pume front in the ast 3 d of samping, with variabe numbers within and outside the pume. Type A carangids were abundant in the pume in the first 2 d of samping, but were rare at a stations after that. Muids showed a more compex tempora pattern, with peaks eary and ate in the samping period, athough they were uniformy abundant at the pume front. Hoocentrid arvae were captured in ow numbers during the first 2 d of samping and then ony in coasta waters. High numbers were recorded during the fina days of samping, both on and outside the front. Pomacentrid arvae were aso captured in ow numbers during the first 2 d of samping. Numbers of this famiy were dominated by catches at the pume front on 22 January. Finay, gobiids showed no cear associations with either station or date. To provide a more powerfu test of the effect of the pume on arva distributions, we pooed sampes through time and then bootstrapped 95% confidence intervas on resuting means. We cacuated 1, bootstrap estimates, using the 15 sampes at each of the three positions inside, outside, and on the pume front (Efron and Gong 1983). Abundances of tota arvae were highest in the pume waters, athough they were not significanty different from the pume front (Fig. 3). Bootstrapped confidence intervas for type A carangids showed that numbers within the pume were significanty higher than in fronta or coasta waters; mean abundance inside the pume was indeed amost an order of magnitude higher than at the other two stations. Muid arvae were significanty more abundant at the pume front than either inside or outside the front, which were not significanty different. Mean numbers of hoocentrid arvae were higher in coasta waters and at the pume front, athough 95% confidence intervas of the mean at the front overapped, abeit ony sighty, with those from inside the pume. Pomacentrid arvae were aso more abundant in coasta waters and at the front; means at both these stations were significanty higher than in the pume. Finay, no spatia differences in mean abundance were detected for gobiid arvae. Convergence zones are formed by a number of hydrographic phenomena in coasta waters. Riverine pumes generate particuary strong convergent veocities, driven by density differences between ighter (in this case warmer and ess saine) pume water and heavier (coder and more saine) coasta water (Garvine 1986). Studies documenting enhanced densities of arva fish at pume fronts have emphasized the passive accumuation of positivey buoyant or surface-seeking organisms (Govoni et a. 1989; Govoni and Grimes 1992). In our study ony the famiy Muidae was significanty more abundant at the front

3 179 Tabe 1. Resuts of Mante and partia Mante tests against spatia and tempora modes, where prob(t) is the probabiity of the nu hypothesis being true obtained from the Mante approximate test. In partia Mante tests, the matrix hed constant is in parentheses. Source r t Prob(t) Station Date Station x date Date x station Tota fish Type A carangids T than either inside or outside the pume. Muids are aso among the most surface-oriented shorefish arvae in the study area (Leis 1991). This distribution pattern woud, therefore, appear adequatey expained by a combination of neustonic arva behavior and surface convergence at the fronta boundary. Aternativey, muid arvae may be activey aggregating in the fronta region. We noted arge amounts of drift agae and fotsam at the fronta boundary, and presettement muids are known to aggregate in the vicinity of such structures (Kingsford 1993). Such interactions need to be considered if the oceanographic features under investigation aso accumuate fish attractors such as fotsam or drifting agae. The significant interaction between station and date in mutivariate anayses presented here emphasized the dynamic nature of the reationship between the pume and ichthyopankton distributions. Some of this variance can be expained by the physica properties of the pume itsef. Riverine pumes in this region are directed offshore by source momentum and are aso defected northward by a combination of the buoyancy differentia between water masses and Coriois force (Woanski and Jones 198 1). Samping positions remained constant with respect to the pume front, athough the geographic position of these stations changed daiy as the front moved offshore. Initiay the pume front was ocated 8 km from the coast, but by the end of the study it was -2 km offshore. Despite the offshore movement of the pume, there was itte evidence of arvae from coasta water becoming entrained in pume waters. Pume sampes custered separatey from front and coasta water sampes over a samping dates. This pattern was not due to a ack of mixing between water masses, as the sainity of the pume increased through time as it moved progressivey offshore (McKinnon and Thorrod 1993). Woanski and Jones (198 1) simiary reported that atera and vertica mixing aong the interface broke down pume integrity over a period of severa days. Rather, it appears that the pume acted to transocate the coasta assembage offshore. Hoocentrids and pomacentrids were initiay present in ow numbers in coasta waters but became abundant both at the front and in the coasta water mass in the atter haf of the samping period. Reativey high numbers of arvae at the front at this time may, then, have been due to the accumuating effect of the pume front as it propagated offshore Muidae 8 -- \ \ 4- -/-- /. 4# I I I I I 1 1 Hoocentridae T Pomacentridae T 7.5 -/ Gobiidae 19 Jan 2 Jan 21 Jan 22 Jan 25 Jan Fig. 2. Mean number of arvae (*SE) from six taxa within the pume (a), at the pume front (), and in coasta water offshore of the pume (C), January Attention paced on inear structures such as the Mississippi River pume may have underestimated the potentia of pume waters to provide enhanced conditions for growth and surviva of fish arvae. Powe et a. (199) found that, contrary to expectations, arva fish captured / b

4 18 Notes Type A carangids 4-3- Gobiidae I wide On Outside 2- I - 1 I I Inside On Outside Fig. 3. Mean and bootstrapped 95% confidence intervas of six taxa at stations 3 km inside the pume front, on the pume front, and 3 km outside the pume front. I at the pume front were in worse condition than those inside and outside the front. As they noted, convergence associated with the front may have served to aggregate arva fish with imited ocomotory abiities and itte chance of surviva. Pume waters in our study apparenty offered enhanced feeding conditions over a much broader area than the pume front. Tota pankton abundance, densities of a number of copepod species, and egg production of a common coasta copepod (A. gibber) were a dramaticay higher in the pume water than in the coasta water mass (McKinnon and Thorrod 1993). Indeed pume environments seem to be characterized by high copepod egg production rates and naupii densities. Dagg and Whitedge (199 1) found arge numbers of copepod naupii in the Mississippi River pume. Naupii densities were simiary high in the Gaspe current, a buoyancy-driven coasta jet that advects estuarine waters into the Guf of St. Lawrence (For-tier et a. 1992). Given that pume waters can cover thousands of square kiometers in the centra region of the Great Barrier Reef, we suspect that the ow-sainity pume may have affected arva fish surviva and recruitment far more than the associated pume front. Austraian Institute of Marine Science PMB No. 3 Townsvie, Queensand 48 1 and Department of Marine Bioogy James Cook University Townsvie, Queensand 48 11, Austraia Austraian Institute of Marine Science Simon R. Thorrod A. David McKinnon 1 Present address: Marine Fish Division, Bedford Institute of Oceanography, POB 16, Dartmouth, Nova Scotia B2Y 4A2. Acknowedgments Financia support to S.R.T. was provided by the Monkman Feowship, Department of Marine Bioogy, James Cook University of North Queensand and an AIMS postgraduate schoarship. Logistic support was provided by AIMS. We thank J. Sharp and H. Layton, skippers of the RV Pegasus, and D. Wiiams,

5 Notes 181 D. IUumpp, and A. Robertson for their support of the work presented here. The comments of two anonymous reviewers consideraby improved the manuscript. This is AIMS pubication 71. References BOWMAN, M. J Estuarine fronts, p In Hydrodynamics of estuaries. CRC. DAGG, M. J., P. B. ORTNER, AND F. AL-Y Wintertime distribution and abundance of copepod naupii in the northern Guf of Mexico. Fish. Bu. 86: , AND T. E. WHITLEDGE Concentrations of copepod naupii associated with the nutrient-rich pume of the Mississippi River. Cont. Shef Res. 11: EFRON, B., AND G. GONG A eisurey ook at the bootstrap, the jacknife and cross-vaidation. Am. Stat. 37: FORTIER, L., M. E. LEVASSEUR, R. DROLET, AND J.-C. THERRIAULT Export production and the distribution of fish arvae and their prey in a coasta jet fronta region. Mar. Eco. Prog. Ser. 85: GARVINE, R. W The roe of brackish pumes in open shef waters, p In S. Skreset [ed.], The roe of freshwater outfow in coasta marine ecosystems. Springer. GOVONI, J. J., AND C. B. GRIMES The surface accumuation of arva fishes by hydrodynamic convergence within the Mississippi River pume front. Cont. Shef Res. 12: , D. E. Hoss, AND D. R. COLBY The spatia distribution of arva fishes about the Mississippi River pume. Limno. Oceanogr. 34: GRIMES, C. B., AND J. H. FINUCANE Spatia distribution and abundance of arva and juvenie fish, chorophy and macrozoopankton around the Mississippi River discharge pume, and the roe of the pume in fish recruitment. Mar. Eco. Prog. Ser. 75: KINGSFORD, M. J. 1993, Biotic and abiotic structure in the peagic environment: Importance to sma fishes. Bu. Mar. Sci. 53: LEIS, J. M Vertica distribution of fish arvae in the Great Barrier Reef agoon, Austraia. Mar. Bio. 19: MCKINNON, A-D., ANDS. R. THORROLD Zoopankton community structure and copepod egg production in coasta waters of the centra Great Barrier Reef agoon. J. Pankton Res. 15: MANTEL, N The detection of disease custering and a generaized regression approach. Cancer Res. 27: POWELL, A. B., A. J. CHESTER, J. J. GOVONI, AND S. M. WARLEN Nutritiona condition of spot arvae associated with the Mississippi River pume. Trans. Am. Fish. Sot. 119: SMOUSE, P. E., J. C. LONG, AND R. R. SOKAL Mutipe regression and correation extensions of the Mante test of matrix correspondence. Syst. Zoo. 35: THORROLD, S. R Meso-scae patterns in the distribution of arva fishes across the centra Great Barrier Reef agoon and reationships with environmenta variabiity. Ph.D. thesis, James Cook University. 27 p. WOLANSKI, E., AND M. JONES Physica properties of Great Barrier Reef agoon waters near Townsvie. 1. Effects of Burdekin River foods. Aust. J. Mar. Freshwater Res. 32: Submitted: 19 May 1994 Accepted: 29 September 1994 Amended: 2 November 1994 Lmno. Oceanogr., 4(I), 1995, , by the American Society of Limnoogy and Oceanography, Inc. Identification of marine invertebrate arvae by means of PCR-RAPD species-specific markers Abstract- We used the poymerase chain reaction (PCR) to randomy ampify poymorphic DNA (RAPD) to identify species-specific genetic markers for five species of gorgonian coras. Using PCR-RAPDs, we can identify adut gorgonians as we as arvae coected in the fied. This technique can be appied to any organism in which identification based on morphoogic traits is difficut. Identification of arvae aows more detaied studies of eary ife histories of marine organisms and studies of the subsequent effects on the dynamics and genetic structure of adut popuations. Unti recenty, studies of marine popuation ecoogy have concentrated on aduts. However, many marine organisms pass through a arva stage before metamorphosis into the adut form, and events in these eary ife-history stages are often critica to understanding the dynamics and genetic structure of adut popuations (Roughgarden et a. 1988; Grosberg and Levitan 1992). Studying adut popuation structure aows inferences to be drawn about the net effects of gene fow (i.e. arva dispersa) and oca seection, but studies of arvae are necessary to fuy characterize the sources of genetic variation in adut popuations (Hedgecock 1986; Grosberg and Levitan 1992). Unfortunatey, techniques for the species-eve identification of most arvae are not avaiabe, making these eary ife-history stages difficut to study in the fied (Hedgecock 1986; Roughgarden et a. 1988; Levin 199). As Grosberg and Levitan (1992, p. 13 1) state, characterizing the mechanisms and patterns of dispersa and demography of marine invertebrate propagues is pagued with the difficuties of foowing very eusive, sma objects in a diute medium across potentiay vast distances. Aozyme and mitochondria DNA restriction fragment ength poymorphism (RFLP) anayses of adut popuations have proven usefu in examining questions of