Deep scattering layers and diel vertical migrations

Transcription

1 Deep scattering layers and diel vertical migrations Deep scattering layers Definition and key features When sonar equipment was first used by oceanographers several mysterious layers were found at different depths in the water column. Sonar works by emitting sound waves that are reflected by objects. These layers were reflecting some of these sound waves. Layers that appeared on sonar readings in this way came to be known as Deep scattering layers (DSLs). Further investigation showed that DSLs are caused by the scattering and reflection of sound waves from groups of organisms particularly fish, euphausiids, and shrimps congregating at specific depths. DSLs look like false sea bottoms on echograms (Figure 2) They were initially believed to be the result of physical phenomena. However, their rhythm provided a clue that they were caused by the movements of animals. The vertical movement of DSLs are often associated with the vertical migration of organisms through the water column. Euphausiids and shrimps have hard exoskeletons made of chitin. This characteristic makes them very effective at reflecting sonar. 74

2 other zooplankton such as heteropods and large copepods can occasionally form sound-reflecting layers too. Sometimes as many as five DSLs can be present during the daytime. These often change depth through the course of a day and may merge. Many organisms migrate from deeper to shallower waters and back again every 24 hours. These are known as diel vertical migrations. Figure 2 An echogram showing day-time deep scattering layers produced by euphausiids (ca m), fish (ca m) and unidentified animals (ca. 175 m) in Saanich Inlet, British Columbia, Canada. Note that the fish show up as discrete dots, whereas the smaller but more abundant euphausiids produce a more even shading pattern. Note also the irregularity of the seafloor, with an abrupt rise from about 225 m to 100 m in the centre of this transect. Diurnal (Diel) Vertical Migration (DVM) Diel vertical migration is a migration pattern through the water column for a variety of planktonic species that is repeated at 24-hour intervals. Typically upwards at night and downward during the day. Figure 3. The depth distribution at different times of day of a vertically migrating copepod. 75

3 Although, by definition, plankton cannot freely move horizontally, they are capable of moving vertically through the water column. The vertical distance traveled over 24 hours varies, generally being greater among larger species and better swimmers. But even small copepods and small thecosome pteropods may migrate several hundred metres ( m) twice in a 24-hour period, and stronger swimmers like euphausiids and pelagic shrimp may travel 800 m or more. Upward swimming speeds of copepods and the larvae of barnacles and crabs have been measured at m h -1 and euphausiids swim at rates of m h -1. Although the depth range of migration may be inhibited by the presence of a thermocline or pycnocline, this is not necessarily so, and an animal may traverse strong temperature and density gradients, as well as considerable pressure changes, during its migration Phytoplankton may migrate through the water column to maximize their exposure to sunlight during the day, and minimize their exposure to predators at night. Zooplankton often migrate with phytoplankton in order to feed off them or to hunt other plant-eating zooplankton. Changes in light levels provide the signal for migrations to begin. It is thought that migrating plankton sense the ambient light in the water around them and rise or fall through the water column to keep it at a constant level. These regions of constant light conditions are known as isolumes. Evidence for this theory is provided by observations that show that o o o o plankton migrate different distances depending on how cloudy the sky is. Natural changes in light intensity which occur seasonally or even daily (e.g. sunny vs. cloudy days; dark nights vs. moonlit nights) can alter the depth ranges inhabited by particular species. Under continuous light in the Arctic summer, migrations may be totally suppressed. A solar eclipse can trigger migrations at the wrong time of day. It will cause animals to begin an upward migration during the day as light intensity decreases. In the laboratory, the timing of migrations may or may not change to conform with experimental alternations of light and dark periods. Direct response to variation in light intensity however, is not sufficient to explain the diurnal pattern of movement. Factors other than light may also play a role in initiating the diel migrations; among those suggested as a causal mechanism is hunger, driving animals upward toward the more productive areas under the protective cover of darkness. There may be an internal biological clock. It is worth to note that such pattern of migration is still poorly understood. 76

4 Migration types Nocturnal migration This is the most common pattern displayed by marine zooplankton. It is characterized by a single daily ascent, usually beginning near sunset, and a single descent from the upper layers which occurs near sunrise. Many midwater zooplankton and fish rise from the mesopelagic to the epipelagic zone at dusk and descend to their previous level at sunrise. These vertical migrations can be detected through the movement of deep scattering layers revealed by sonar. Reverse migration It is the least common pattern. It is characterized by a surface rise during the day and a night-time descent to a maximum depth. In some epipelagic communities, phytoplankton and/or zooplankton descend through the epipelagic zone at night and ascend during the day. Twilight (double) migration It is marked by two ascents and two descents every 24 hours. There is a sunset rise to a minimum night-time depth, but during the night there is a descent called the midnight sink. At sunrise, the animals again rise toward the surface, then later descend to the daytime depth. 77

5 Some mesopelagic zooplankton rise to the epipelagic zone at dusk, descend during the night, rise again near dawn, and then descend to their daytime position. Hypothesis of Diurnal Vertical Migrations and their Validity Many hypotheses have been advanced to answer the question, why so many species should show this behavior?, but it may not be realistic to insist on a universal mechanism governing diel vertical migration in all species. It is important to recognize that the hypotheses discussed below may not be mutually exclusive, and that each may be more applicable to some species than others. 1. Strong light hypothesis. Zooplankton are adversely affected by strong light and therefore leave surface waters during the day. Validity: some animals migrate to depths far greater than those at which surface intensity can be damaging. 2. Phytoplankton recovery hypothesis. Zooplankton exploit the phytoplankton at night but dive to allow the phytoplankton to photosynthesize and to recover during the day, so that they can be exploited again the following night. Validity: cooperation among zooplankton organisms would be required for a complete recovery to occur. Also, natural selection favors individuals, and those that would "cheat" by remaining in the surface waters could take advantage of the remaining phytoplankton. Furthermore, experimental manipulations of food concentrations produce conflicting results depending on the species. In some cases, low food levels suppress vertical migration; in other examples, the reverse is true. 3. Surface mixing hypothesis. The zooplankton move downward during the day, in the hope of returning to surface waters that have been driven from another locale by the winds and currents. Thus they encounter a new feeding area of surface waters each time they 78

6 ascend which may contain more phytoplanktonic food than the area occupied the night before. Validity: surface mixing hypothesis has as the same logical problem as phytoplankton recovery hypothesis. 4. Predation avoidance hypothesis. Predators (e.g., fish, diving birds) use vision to capture prey, so zooplankton leave the surface waters during the day to avoid being seen. They return at night to the surface waters to exploit the phytoplankton. It has been shown, for example, that diel migrations in several zooplankton species may become more pronounced when predatory fish are more abundant. Validity: A 45-year-long study of Metridia lucens in the North Atlantic has shown that the length of time this copepod was present near the surface varied seasonally, being shorter in the summer when nights are shorter. However, (1) when food was most abundant during the spring, the animals remained longer at the surface than was predicted from length of daylight; the importance of obtaining food when it is most abundant seems to override the importance of predator avoidance at this time. (2) Copepods in Dabob Bay, Washington, actually perform a reverse vertical migration, spending daylight hours in the surface waters and nighttime at depth. This seems to be a response to arrow worm predators, which themselves carry out a typical vertical migration (upward at night, downward during the day). (3) Furthermore, many zooplankton migrate far deeper than is needed to avoid predators. (4) Also, many vertically migrating species bioluminesce at night, which would seem only to attract predators. (5) Another objection is that members of species that are relatively invisible (e.g., transparent gelatinous zooplankton) are among the strongest in vertical migration. 5. Energy conservation hypothesis. It is energetically advantageous to spend the non-feeding time day in colder, deeper water, where metabolic rate and energy demands are less. The animals come up at night to feed on phytoplankton or on other zooplankton that come to the surface. Validity: In Gatun Lake, Panama, the calanoid copepod Diaptomus gatunensis shows a diurnal vertical migration, despite the lake has no vertical gradient in temperature, a fact that eliminates the energy conservation hypothesis from consideration, at least in the lake. The most valid (acceptable) hypothesis The energy conservation hypothesis depends on the increase of metabolic rate with increasing temperature in poikilotherms. By spending some time in the cooler deep waters, an animal can save energy. This gain would have to be balanced against the energy required to migrate and against the time lost from feeding at the surface. Apparently, the energy expended in downward and upward movements is quite modest and not a consideration, generally only a few percent of basic metabolic energy. Dawidowicz and Looser measured reproduction and growth rate in moving and in stationary water fleas and found no differences 79

7 between the two groups. McLaren calculated that copepods should have an energetic advantage in spending time in deeper waters where temperatures are cooler and metabolic costs lower. This energetic advantage would be realized in the production of additional eggs and greater potential population growth. Copepods often retire to deeper waters after the spring peak of phytoplankton production. It may be that this is an adaptation to lower energy expenditure that comes into play once the surface food source is no longer abundant. Consequences of Diel vertical migration Diel vertical migration has several consequences that are biologically and ecologically important. 1. Samples from same depths taken during day and night will differ in species composition and total biomass 2. Enhances genetic mixing Since all individuals of a species do not migrate at precisely the same time and to the same depths, a population will eventually lose some individuals and gain others. This mixture of individuals from different populations enhances genetic mixing and is especially important in species of limited horizontal mobility. 3. Increases and hastens the transfer of organic materials produced in the euphotic zone to deeper areas of the sea The ladder-like series of migrating organisms (Figure 8) plays an important role in marine food chains. Animals capture prey at shallower depths and transport it downwards either as their body mass or fecal products. The active vertical transport of organic materials, either in the form of the animals themselves or in their faecal pellets and other wastes is significantly faster than the passive sinking of organic particles. Figure 8 A schematic illustration of the diel migration patterns of pelagic shrimps living in different vertical zones. Dotted and hatched areas indicate the depth ranges of the main day and night concentrations, respectively, of the different groups. Each group (1-7 ) is a composite of different species that occupy similar depth ranges. 80

8 Seasonal vertical migrations A seasonal vertical migration (SVM) is a migration pattern through the water column that is triggered by a change in the seasons. In some species, SVM may be associated with breeding cycles and changing depth preferences of different stages in the life cycle. SVMs are most common in species living in polar or temperate waters where there is a large variation in climate between seasons. It also undertaken by species living in upwelling regions. Polar regions In polar regions, phytoplankton production is highly seasonal, being greatest in spring and summer. The zooplankton that feed on them are only found in surface waters during spring and summer. In autumn and winter they remain at cooler depths to conserve energy. Life cycle migrations Some species migrate to different depths in the water column at different stages of their life cycle. These life cycle stages often correspond to seasons. For example, among free-living copepods, different stages of the life cycle are commonly found at different depths, and are associated with different seasons of the year. In inshore waters off the western coast of Canada, Neocalanus plumchrus adults do not feed, and they overwinter at about m depth where the eggs are laid between December and April (Figure 9). Figure 9 A schematic diagram of the life cycle of the copepod Neocalanus plumchrus in coastal waters off British Columbia, Canada. The depth distributions of the eggs, larvae (nauplii I-VI and copepodites l - V ) and adults (copepodite VI) are shown over the course of one year7c, copepodite; N, nauplius. 81

9 The eggs float toward the surface, and nauplii hatch and develop at intermediate depths. Nauplii are present in near-surface waters from February to April, and the population matures to the copepodite V stage during March to June when primary productivity is highest. By early June, stage V individuals contain large amounts of lipids accumulated from feeding on phytoplankton, and they begin to migrate to deeper waters where they will subsist on this stored fat reserve. There they mature to the adult stage VI, mate, and lay eggs during the winter. A similar pattern of vertical migration associated with different reproductive stages takes place in Neocalanus cristatus, a large copepod also common to the North Pacific: adults are present between 500 m and 2000 m, and spawning occurs in deep water; younger stages move upward and live mostly above 250 m. The Antarctic krill (Euphausia superba) also undergoes extensive depth changes during its life cycle. The eggs of the krill are deposited in surface waters but sink rapidly to depths of m, where they hatch. The larvae then gradually float and swim to the surface where development is completed, and juveniles and adults are found at or very near the surface. Importance of vertical migration (diel and seasonal vertical migrations) The migrations usually result in animals being within the productive surface waters. Seasonal migrations for example result in young animals being within such productive waters at a time when they can obtain sufficient quantities of food for growth. In temperate waters, as production declines in the surface waters during summer and fall, late larval stages or sexually mature adults move to deeper waters. Here, in colder and unproductive waters, they may enter a state called diapause in which their metabolism slows and they do not feed. Instead, they subsist on energy reserves built up during their stay in the surface zone. Place migrants in currents that are moving in different directions and at variable speeds. Despite this, populations of marine zooplankton do persist in their own characteristic geographical regions. Retain animals in favorable habitats. A distinctive pattern of seasonal vertical migrations in a species may ensure retention within an appropriate habitat, or within a productive upwelling area. Diel migrations may similarly retain animals in favorable habitats. 82

10 Seasonal changes in diel vertical distribution Figure 10 shows both the seasonal and diel vertical migrations in two species of North Atlantic herbivorous copepods, Calanus helgolandicus and C. finmarchicus. Figure 10 Seasonal changes in the day-time (white) and night-time (black) vertical distribution of copepodite stages V and VI of two species of copepods, Calanus helgolandicus (a) and C. finmarchicus (b), in the Celtic Sea. Numbers in each plankton haul are plotted in 5 m depth intervals as percentages of total numbers (n) present in the haul. Temperature profiles are shown for the day hauls and apply to both species. During winter in the Celtic Sea, copepodites V and VI of both species are distributed fairly uniformly from the surface to about 100 m. and there is little difference between day and night-time distributions. In spring (April), both species begin to concentrate in shallower depths, and they display diel vertical migrations. In July and August, the thermocline becomes well established and the two species show a clear separation in their distributions. Calanus helgolandicus continues to develop in the warmer surface zone and to display diel migration, but C. finmarchicus moves deeper into cooler water beneath the thermocline and shows little difference in day and night depth preferences. By late September, both species reside in water deeper than 40 m during the day, and C. helgolandicus maintains its strong vertical movement toward the surface at night. 83