Spatial dynamics of animal‐mediated nutrients in temperate waters

Abstract

Consumer‐mediated nutrient dynamics (CND), through which animals' metabolic waste products fertilize primary producers, drive variability in nutrient availability in tropical waters. This variability influences primary productivity and community functioning. Yet, examinations of CND as a driver of nutrient variability in temperate marine ecosystems are limited. Therefore, we assessed the existence and drivers of variation in CND in temperate waters at meso, small, and fine spatial scales. We quantified the occurrence of 48 fish and 92 macroinvertebrate taxa and measured in situ ammonium at 27 northeast Pacific rocky reefs for 3 yr and 16 kelp forests of varying density for 1 yr. Ammonium concentrations ranged from 0.01 to 2.5 μ M across rocky reefs separated by tens of kilometers. The relationship between animal abundance and ammonium among sites was mediated by water flow, where flood tides seemed to “wash away” the effect of nutrient regeneration by animals, although enrichment was possible on ebb tides. Ammonium concentration was significantly greater within than outside of kelp forests, a difference that increased with kelp biomass, tidal exchange, and, to a lesser degree, animal biomass. Caging experiments revealed that fine‐scale (~ 2 m) ammonium variability and nutrient enrichment were only possible under low‐flow conditions. Our results suggest that CND drives nutrient variability at scales ranging from two meters to over 20 km, acting on a finer scale than allochthonous nitrogen sources such as upwelling. Therefore, CND are implicated as a previously overlooked driver of spatial variation in primary productivity in temperate marine systems.

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