Consumers

Grazers

Grazers are animals that eat algae, bacteria, and other small organisms. They are consumers — they cannot make their own food from light and must eat to survive. All grazers share several common traits: their activity slows when oxygen or salinity falls outside tolerable ranges; they maintain fixed body composition ratios (C:N and N:P) by excreting excess nitrogen as ammonium and excess phosphorus as phosphate; they die of starvation when food intake is insufficient; they suffer toxicity from high ammonia levels; and they respire, consuming oxygen and producing CO₂. Feeding follows Holling Type II functional response kinetics: intake rate increases with food availability but saturates at a maximum. Grazers are less active at night and when oxygen is low. Undigested food is egested as fecal pellets that become detritus.

Sloppy feeding. When a consumer captures and handles prey, some cell contents spill into the water before being ingested — cells rupture, cytoplasm leaks out, fragments break off. About 10% of all egested material dissolves directly into the water as dissolved organic matter (DOM) rather than forming particulate fecal pellets (Moller 2005). Of that dissolved fraction, 95% becomes labile DOM (cytoplasmic contents like amino acids and sugars that bacteria consume rapidly) and 5% becomes refractory DOM (cell-wall fragments and recalcitrant polymers that turn over slowly). This 10% rate applies uniformly to all consumers. Sloppy feeding is ecologically important because it short-circuits the detritus pathway: instead of carbon and nutrients being locked in fecal pellets that must settle and decompose, a fraction goes directly to dissolved organics that bacteria can use immediately. In systems with active grazing, sloppy feeding becomes a significant source of labile DOM that fuels the microbial loop.

Respiratory quotient. When a consumer respires, it consumes O₂ and produces CO₂, but not necessarily in a 1:1 ratio. The respiratory quotient (RQ) is the ratio of CO₂ produced to O₂ consumed, and it depends on the biochemical composition of the substrates being catabolized. Lipid-rich catabolism gives a low RQ (~0.7), carbohydrate catabolism gives RQ = 1.0, and protein catabolism falls in between (~0.8). Most consumers have an RQ of 0.85, reflecting the mixed protein-and-lipid diet typical of crustacean zooplankton. The two protist consumers — ciliates and nanoflagellates — have a slightly higher RQ of 0.90, because protists catabolize a broader mix of substrates including carbohydrates from bacterial cell walls and cytoplasmic contents, shifting their respiratory gas exchange closer to 1:1. The RQ affects the dissolved CO₂ and O₂ budgets: a higher RQ means more CO₂ returned to the water per unit of oxygen consumed, which in turn influences pH (more CO₂ lowers pH) and the balance between photosynthetic carbon supply and respiratory carbon return.

Oxygen-limited carbon excretion. When dissolved oxygen is too low for a consumer to aerobically metabolize all the carbon it has assimilated, a surplus of unmetabolized carbon accumulates. What happens to this excess depends on the species. Most consumers (Copepods, bladder snail, Neocaridina, ostracod, rotifer, ciliate, nanoflagellate) excrete it as simple organic waste products analogous to lactate or ethanol from anaerobic fermentation. Bacteria rapidly mineralize these waste products into dissolved inorganic carbon (CO₂ and bicarbonate). Daphnia instead egest the excess as organic particles that join the suspended detritus, reflecting their tendency to package undigested material into fecal matter rather than excrete dissolved waste. This distinction matters most during hypoxic episodes: species that excrete dissolved waste return carbon to the inorganic pool immediately (available for photosynthesis), while species that produce particulate waste create a delayed recycling pathway that must pass through bacterial decomposition first.

Predators

Predators capture and consume other animals rather than algae or detritus. Unlike grazers, predators are gape-limited (they can only handle prey of certain size and morphology), so they reshape the prey community structure rather than simply reducing total biomass — typically driving a trophic cascade where suppressed grazers allow algae to rebound.

Hydra

Microzooplankton

Microzooplankton occupy the microbial loop, linking bacteria and dissolved organic matter to larger consumers.