The Structure of Marine Communities Over Time: Long and Short-Term Changes
Conditions and organisms change on marine environments at a wide range of scales, from minutes to thousands of years (Haury et al., 1978). Even at great depths near the bottom where temperature and the chemical milieu are rather constant, there are pulses of materials raining down from the seasonal cycles of production near the surface (cf. Section 10.1121). Presumably this is why seasonal breeding takes place even in benthic organisms in the otherwise very constant deep sea (Grassle and Sanders, 1973; Rokop, 1974, 1977). The magnitude of production at the surface also varies from year to year, so that bottom fauna are exposed to variation over years as well as months.
KeywordsSalt Marsh Mixed Layer Seasonal Cycle Great Barrier Reef Vertical Migration
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- Recall from Section 4.4 that competition for food was evident in young fish so they must limit their food supply. Other predators of copepods such as chaetognaths may also restrict zooplankton grazing. Noctiluca are algal feeders but can also effectively eat eggs of copepods such as Acartia (Ogawa and Nakahara, 1979). In the Sea of Japan maximum densities of such predators coincide with low peaks of copepod abundance. The abundance of adult fish, as we might expect from Chapter 9, seem unrelated to events lower down in the food web.Google Scholar
- We have seen that this is true for coastal waters in general (Section 2.23). In some estuaries, as those in the North Sea near Holland, silicates may be the limiting nutrient for algal growth at the head of the estuary (Gieskes and Kraay, 1975).Google Scholar
- Menhaden usually feed on phytoplankton, but in summer at the head of Narragansett Bay the dominant algae are small flagellates. These are too small for menhaden and therefore the fish switch feeding methods and feed raptorially on the abundant zooplankton (Kremer and Nixon, 1978) .Google Scholar
- After the construction of the Aswan High Dam in the 1960s, this whole pattern was disrupted, and coastal fisheries reduced because of the lack of nutrient input into the nutrient-poor Eastern Mediterranean.Google Scholar
- In the North Sea, for example, only 0.03 g cal cm-2 min-1 are needed to start the spring bloom (Gieskes and Kraay, 1975).Google Scholar
- These are high values compared to other oceanic waters (cf. Section 11.221, Fig. 2–18, and Table 2–3).Google Scholar
- We have seen that current views on how copepods feed suggest that passive sieving through appendages is less likely than previously thought (Chapter 6). In spite of this, intersetule distances may still have some role in feeding on very small particles. N. plumchrus and N. cristatus have relatively large feeding appendages with very closely spaced setules, perhaps an adaptation for a diet of the small cells usually found in surface waters of the subarctic Pacific. N. plumchrus has intersetule distances of 1.5–7.2 µm. In spite of the very large size of this copepod, the 1.5 µm is the smallest such distance for the many species surveyed by Heinrich (1963). For comparison, here are ranges of intersetule distances for a few other suspension feeding herbivores: N. cristatus: 1.8–9.6; Undinula darwini: 3.0–8.4; Pseudocalanus elongatus: 1.8–7.2; Eutideus giesbrechti: 4.2–6.6. Raptorial species, probably mainly predaceous, include Eucheta marina: 16.8–19.2; Metridia pacifica: 2.4–21.6; Labidocera acutifrons: 12.0–27.6; Acartia longiremis: 9.6–19.2.Google Scholar
- Below-ground plant parts store materials over winter and can exceed the weight and production of above-ground parts in marsh plants throughout the year (Valiela et al., 1976; Good et al., 1982; Kistritz et al., 1983).Google Scholar
- In eelgrass there are also important shading relationships between higher plants and epiphytes, but in this case it is the epiphytes that reduce eelgrass production as they grow over the surface of eelgrass leaves (McRoy and McMillan, 1977).Google Scholar