Plankton Food Webs

The most important elements for phytoplankton growth are the macronutri-ents nitrogen (N) and phosphorous (P) and, for diatoms, silica (Si). Phytoplankton cells take up dissolved forms of C, N and P across their cell surfaces in an atomic ratio of 106C:16N:1P (the Redfield ratio). Sometimes the atomic ratios of dissolved nutrients in the water column are different to those required for phytoplankton growth. This provides an important signal to managers and researchers. N: P atomic ratios that are much higher than 16 (say, 25-30) suggest that P limitation of algal growth is occurring, which means that the lack of phosphorous is preventing further algal growth. Alternatively, a ratio of less than 10 would imply N-limited growth.

While phytoplankton growth in freshwater systems is generally P limited, growth in estuarine and oceanic environments is commonly N (and at times also Si) limited. Phytoplankton cells require external sources of other inorganic nutrients, in particular trace metals and minerals (Fe, Mg, Zn, Na, Ca, Mn and others) and vitamins (thiamine, biotin and B^). These are needed in much lesser quantities and are generally assumed (wrongly at times) to be in sufficient quantities for growth.

In some regions of the world's oceans, phytoplankton cells have access to relatively high levels of N and P yet exhibit low biomass (generally determined by chlorophyll-a concentration). A series of elaborate experiments in the Equatorial Pacific demonstrated that this 'high-nutrient, low-biomass' phenomenon was due to iron limitation (Behrenfeld et al. 1996, Timmermann et al. 1998).

In areas of low phytoplankton productivity, most of the phytoplank-ton growth is sustained through 'regeneration' of nutrients. This happens when organic matter (for example, faecal pellets and dead and decaying material) is remineralised to dissolved inorganic nutrients via microbes in the plankton. 'New' production occurs in response to external nutrient inputs (catchments, rivers, atmosphere, and so on) or when turbulent diffusion allows deep water nutrients to cross the thermocline (nutricline) into the surface mixed layer. The ratio of new to regenerated production is referred to as the fratio - the lower the fratio, the greater the dependence on regeneration of nutrients via microbes. Although used as an index of trophic status of an area, the f ratio can vary greatly over time (Platt et al. 1992).

Grazers represent an essential trophic pathway for the transfer of organic carbon from phytoplankton to fish, and they contribute to the nutrient pool by excreting faecal pellets that are either recycled within the water column or used by bottom feeders. Nutrient recycling is also assisted by the 'sloppy feeding' or partial ingestion of cells by herbivorous zooplankters (such as copepods), which results in the release of nutrient-rich cell sap following handling and rupture of captured cells.

Trophic transfer, however, is no longer understood simply as materials and energy passing through producers and a series of consumers in a simple linear chain (the classical food chain). The traditional model of a short marine food chain (phytoplankton ^ copepod ^ fish) became obsolete following recognition of the trophic importance of bacterioplankton and protozoans in marine waters (Malone 1971; Williams 1981). It is now accepted that a significant proportion of phytoplankton production is not consumed directly by zooplankton grazers, but is cycled by the microbial community ('microbial loop') before it becomes available to consumers.

The primary organisms involved in the recycling activities of the microbial loop (Figure 2.3) are water-column bacteria, heterotrophic flagellates and ciliates. One of the roles of the bacteria is to break down organic molecules contained in non-living particulate organic matter (POM) and dissolved organic matter (DOM) derived from living cells, faecal pellets and dead and decomposing bodies. The bacteria convert organic matter to dissolved inorganic nutrients (DIN), such as nitrogen, phosphorus and potassium, which are then available for rapid uptake by phytoplankton. The bacteria are consumed by protozoans (ciliates and nano-flagellates), which are in turn food sources for other zooplankton.

The recycling of POM by the microbial loop also serves to reduce the sedimentation of faecal matter and detritus. This is particularly important in warm, low-nutrient waters, where microbes rapidly and efficiently recycle materials and thus limit the sinking of large amounts of organic matter to

Piscivorous fish & other predators

Planktivorous fish, carnivorous zooplankton

Piscivorous fish & other predators

Planktivorous fish, carnivorous zooplankton

Zooplankton grazers

'Microbial Loop'

Zooplankton grazers

Phytoplankton

^^ f Ciliates

Nano "T"

2-20 | m

Pico

0.2-2 |m / v * J

Figure 2.3 Generalised food web showing classical food chain (left side) and microbial loop (right side), with arrows showing trophic pathways, flow of particulate and dissolved organic matter (POM, DOM) in excretory products and dead organisms (dashed arrows), and flow of dissolved inorganic nutrients (DIN) to phytoplankton. Het. = Heterotrophic.

POM/DOM

Bacteria

Het. Nano Flagellates

Figure 2.3 Generalised food web showing classical food chain (left side) and microbial loop (right side), with arrows showing trophic pathways, flow of particulate and dissolved organic matter (POM, DOM) in excretory products and dead organisms (dashed arrows), and flow of dissolved inorganic nutrients (DIN) to phytoplankton. Het. = Heterotrophic.

the bottom. In cold waters - and during the winter months in many temperate regions - microbial activity is suppressed. The effects are that most of the carbon reaches higher trophic levels directly via the grazing activities of zooplankton, and a large fraction of the carbon fixed during photosynthesis sinks to the bottom where it is then used by benthic communities.

Numerous feeding strategies are employed by small zooplankton (ciliates and flagellates) including herbivory, carnivory and omnivory. But a strategy commonly used by many is 'mixotrophy' - a feeding strategy that combines characteristics of both autotrophs (which make their own food via photosynthesis) and heterotrophs (which ingest food). Numerous species of ciliates that are known to exhibit mixotrophy contain large numbers of chloroplasts (light-harvesting organelles) sequestered from ingested

Figure 2.4 Mixotrophic ciliate with numerous chloroplasts (organelles containing light-harvesting pigments) sequestered from ingested algal cells. (Cell diameter 10-20 ^m.)

phytoplankton (Figure 2.4). They derive nutrition from both the direct ingestion of food and by the carbohydrates made by the sequestered pho-tosynthetically active chloroplasts (Stoecker 1987). This nutritional strategy offers great survival and competitive advantages, especially in environments where food resources are highly variable.

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