Characteristics, dynamics and significance of marine snow

Coral atoll communities are of special interest to marine biologists since they flourish in tropical waters characterized by low productivity (cf. Stoddart 1969). One of the aims of research in coral reef biology is to discern and describe quantitatively the adaptations that permit reef communities to exploit a marginally suitable habitat. Vital to this aim is an understanding of the nutrition of corals themselves since these lime-secreting cnidarians are the dominant life forms of many reefs and one of the principal reef builders. The general topic of coral nutrition has been reviewed recently (Muscatine 1973, Taylor 1973a) and serves as a convenient point of departure. The present discussion considers the potential nutritional capabilities of corals as mutualistic symbioses and the extent to which specific organismic and ecological manifestations of these capabilities have been described and quantified.

Reef Corals: Mutualistic Symbioses Adapted to Nutrient-Poor Environments

Rapid mass sinking of cells following diatom blooms, observed in lakes and the sea, is argued here to represent the transition from a growing to a resting stage in the life histories of these algae. Mass sinking is of survival value in those bloom diatoms that retain viability over long periods in cold, dark water but not in warm, nutrient-depleted surface water. Mechanisms for accelerating sinking speed of populations entering a resting or “seeding” mode are proposed. Previously unexplained features of diatom form and behaviour take on a new meaning in this context of diatom seeding strategies. Diatoms have physiological control over buoyancy as declining growth is accompanied by increasing sinking rates, where the frustule acts as ballast. Increased mucous secretion in conjunction with the cell protuberances characteristic of bloom diatoms leads to entanglement and aggregate formation during sinking; the “sticky” aggregates scavenge mineral and other particles during descent which further accelerates the sinking rate. Such diatom flocs will have sinking rates of ∼ 100 m d-1 or more. This is corroborated by recent observations of mass phytoplankton sedimentation to the deep sea. This mechanism would explain the origin of marine snow flocs containing diatoms in high productivity areas and also the well-known presence of a viable deep sea flora. That mortality is high in such a seeding strategy is not surprising. A number of species-specific variables pertaining to size, morphology, physiology, spore formation and frustule dissolution rate will determine the sinking behaviour and thus control positioning of resting stages in the water column or on the bottom. It is argued that sinking behaviour patterns will be environmentally selected and that some baffling aspects of diatom form and distribution can be explained in this light. Rapid diatom sedimentation is currently believed to be mediated by zooplankton faecal pellets, particularly those of copepods. This view is not supported by recently published observations. I speculate that copepod grazing actually retards rather than accelerates vertical flux, because faecal pellets tend to be recycled within the surface layer by the common herbivorous copepods. Egestion of undigested food by copepods during blooms acts as a storage mechanism, as ungrazed cells are likely to initiate mass precipitation and depletion of the surface layer in essential elements. Unique features of diatoms are discussed in the light of their possible evolution from resting spores of other algae. An evolutionary ecology of pelagic bloom diatoms is deduced from behavioural and morphological characteristics of meroplanktonic and tychopelagic forms. Other shell-bearing protistan plankters share common features with diatoms. Similar life-history patterns are likely to be present in species from all these groups. The geological significance of mass diatom sinking in rapidly affecting transfer of biogenic and mineral particles to the sea floor is pointed out.

Role of sinking in diatom life-history cycles: ecological, evolutionary and geological significance

A pelagic food web model was formulated with the goal of developing a quantitative understanding of the relationship between total production, export production, and environmental variables in marine ecosystems. The model assumes that primary production is partitioned through both large and small phytoplankton and that the food web adjusts to changes in the rate of allochthonous nutrient inputs in a way that maximizes stability, i.e., the ability of the system to return to steady state following a perturbation. The results of the modeling exercise indicate that ef ratios, defined as new production/total production = export production/total production, are relatively insensitive to total production rates at temperatures greater than ∼25°C and lie in the range 0.1-0.2. At moderate to high total production rates, ef ratios are insensitive to total production and negatively correlated with temperature. The maximum ef ratios are ∼0.67 at high rates of production and temperatures of 0°−10°C. At temperatures less than ∼20°C, there is a transition from low ef ratios to relatively high ef ratios as total production increases from low to moderate values. This transition accounts for the hyperbolic relationship often presumed to exist between ef ratios and total production. At low rates of production the model predicts a negative correlation between production and ef ratios, a result consistent with data collected at station ALOHA (22°45′N, 158°W) in the North Pacific subtropical gyre. The predictions of the model are in excellent agreement with results reported from the Joint Global Ocean Flux Study (JGOFS) and from other field work. In these studies, there is virtually no correlation between total production and ef ratios, but temperature alone accounts for 86% of the variance in the ef ratios. Model predictions of the absolute and relative abundance of autotrophic and heterotrophic microorganisms are in excellent agreement with data reported from field studies. Combining the ef ratio model with estimates of ocean temperature and photosynthetic rates derived from satellite data indicates that export production on a global scale is ∼20% of net photosynthesis. The results of the model have important implications for the impact of climate change on export production, particularly with respect to temperature effects.

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Temperature effects on export production in the open ocean

Concentrations of carbon, nitrogen and phosphorus were determined in particles that passively sank into multi-replicate collectors set at 50, 250, and 700 m in coastal waters, and 75, 575, and 1050 m in the open ocean. Fluxes as high as 36, 4.1, and 0.19 mmoles of C, N, and P m−2 day−1 were observed at 50 m under coastal upwelling conditions; at 700 m, upwelling period fluxes (9.6, 0.9, and, 0.053 mmoles of C, N and P m−2 day−1) exceeded those measured at 50 and 75 m when samplers were set under low productivity surface waters. 210Pb flux estimates were made on coastal trap particulates. The resulting values were close to the expected and suggest that overall flux estimates are representative of those occuring in the environment. Atomic ratios of C:N:P under upwelling conditions were similar to values reported for living plankton (∼180:18:1), while in the open ocean, atomic ratios of C and N in relation to P were markedly higher (400 to 900:30:1). Fecal pellet fluxes were two orders of magnitude higher under upwelling conditions (∼1 to 3 × 105m−2 day−1) than those in the open ocean (∼1000 m−2 day−1). Quantities of passively sinking particulate C, N, and P appeared to be equal to or in excess of the amounts required to meet the nutritional needs of the mid-water zooplankton. Rates of change for C, N, and P and inferred rates of oxygen change varied widely in relation to surface productivity. For example, oxygen utilization rates were as high as 790 μll−1 yr−1 in near-surface waters under upwelling conditions and as low as 4.4 μll−1 yr−1 at mid-depth in the open ocean. Our rates of change, determined by direct measurement, generally agree with previously published estimates from mathematical models.

Fluxes of particulate carbon, nitrogen, and phosphorus in the upper water column of the northeast Pacific

Per unit weight, marine protozoa excrete dissolved phosphorus one to two orders of magnitude faster than marine microcrustaceans and several orders of magnitude faster than marine macrofauna. Protozoa may therefore be responsible for a major fraction of faunal nutrient excretion even though present only as a minor fraction of the faunal biomass. Regeneration of dissolved inorganic phosphate from organic detritus proceeds faster and more completely in the presence of bacteria and ciliates or colorless flagellates than in the presence of bacteria alone. An increased consumption of organic detritus in the presence of protozoa is performed by bacteria, which are kept in a prolonged state of “physiological youth” by grazing protozoa. Present results indicate that it is the protozoa that produce, via excretion, the greater regeneration of nutrients from organic detritus. By constituting a nutrient source for protozoa, bacteria are indirectly involved in this increased

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1965.10.3.0434

Influence of marine protozoa on nutrient regeneration1

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1966.11.2.0191

Composition and nutritive value of fecal pellets of a marine crustacean1

A marked increase in the concentration of suspended particulate organic aggregates occurs in oceanic water as it crosses the windward coral reef at Eniwetok Atoll and enters the lagoon. Evidence is presented indicating that these aggregates consist of mucus released by corals. Export of mucus into the lagoon by the reef community was estimated at 20 mg m-’ hr-l. This accounts for about 2.0% of total reef community production and is equivalent to about 40% of coral respiration. Field observations and experimental evidence suggest that aggregates of mucus may be an important food source for zooplankton in the vicinity of coral reefs.

https://aslopubs.onlinelibrary.wiley.com/doi/epdf/10.4319/lo.1967.12.2.0189

Ecology of organic aggregates in the vicinity of a coral reef1

Over 15 days of continuous monitoring, the mean nocturnal concentration of drifting net zooplankton in a coral reef community on the Bermuda platform was 0.83 mg m−3. The mean respiration rate of reef-building corals at the study site was calculated to be 6.4 g O2 day−1 m−2 of reef bottom. Using these data we show that the energy requirement of these corals was more than an order of magnitude greater than the energy they could obtain by capturing the sparse zooplankton in the surrounding waters.

The question follows, why, if corals can exist in the absence of energetically significant quantities of zooplankton, do their elaborate anatomic modifications for catching zooplankton persist? Evidence indicates that corals cannot obtain their phosphorus requirements from solution. We therefore suggest that relatively small amounts of zooplankton may be important as a source of phosphorus and perhaps of other essential nutrients for both the coral and its algal symbionts.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1970.15.4.0579

R. E. Johannes

Papers

Influence of marine protozoa on nutrient regeneration1

Composition and nutritive value of fecal pellets of a marine crustacean1

Ecology of organic aggregates in the vicinity of a coral reef1

The role of zooplankton in the nutrition of some scleractinian corals1

Measuring Organic Matter Retained by Aquatic Invertebrates