Biotic controls of nutrient cycling

Nutrient uptake and cycling is expected to vary directly in response to biotic demand by primary producers and heterotrophic microorganisms in biofilms and other sites of high biological activity, and thus ultimately be influenced by the environmental factors that control rates of primary and microbial production. More productive systems should cycle nutrients at higher rates, owing to higher uptake rates as well as higher rates of regeneration due to consumption, excretion, and egestion and subsequent mineralization. Animal consumers such as herbivores can stimulate rates of production by increasing the turnover of producers and regenerating nutrients that help to meet producer demand. The dissimilatory activities of nitrifying and denitrifying bacteria change the concentrations of various forms of inorganic N, enhancing or limiting its bioavailability. The efficiency with which stream ecosystems are able to utilize, retain, and recycle nutrients is largely determined by environmental factors that influence uptake and assimilation by the biota and, in the case of N, the various biologically mediated transformations that determine its chemical form.

11.3.3.1 Assimilatory uptake

The influence of the benthic algal community upon nutrient uptake is nicely illustrated by Grimm's (1987) study of successional events in Sycamore Creek, Arizona. Following a flood that eliminated virtually all of the biota, biomass accumulated rapidly and then more slowly as the system acquired a thick periphyton mat and high densities of invertebrates. Measurements of hydrologic inputs and outputs of N and N storage in the biota documented substantial retention of inorganic N within the 90 m reach due to N uptake and accumulation in living tissue. In a subsequent study of the same system using nitrate and chloride additions to measure uptake length, Marti et al. (1997) showed that nitrate SW was short relative to other published studies, consistent with evidence that N was limiting in this desert stream. Nitrate uptake length in Sycamore Creek doubled after a modest midsummer flood, but the system returned to low flow conditions within hours and algal recovery occurred within weeks. Using streamwater N concentrations sampled during a much larger flood studied by Fisher et al. (1982), Marti et al. determined that N retention is minimal immediately after a flood, but recovers quickly as the biota become reestablished (Figure 11.9). These studies clearly show that nutrient retention depends on the development of the algal mat, which in turn is strongly influenced by disturbance frequency in this desert stream. The composition of the mat was also found to be important, as N retention was significantly lower when N-fixing cyanobacteria made up greater than 10% of periphyton.

Rates of uptake of ammonium and nitrate and the relative importance of assimilatory and dis-similatory processes vary widely among studies that have used 15N tracer releases in small streams. In a comparative study of 11 headwater streams from biomes throughout North America, removal of ammonium was attributed primarily to assimilation by autotrophs, bacteria, and fungi, and sorption to sediments (70-80% for

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