Empirical measurements of ammonium excretion in kelp forest fishes: Effects of body size, taxonomy and trophic guild

Fishes and other consumers excrete metabolic waste products, including
dissolved nutrients rich in nitrogen, which is an essential nutrient for
primary production. Relatively little is known about the magnitude and
variability of nutrients excreted by fishes in kelp forest ecosystems and
whether consumer-derived nutrients are important for supporting kelp
productivity. In this study, the supply of ammonium (NH4+) excreted by the
dominant fishes (30 species representing ~85% of total fish biomass) was
investigated on nearshore rocky reefs in California. Using rapid field
incubations, the amount of excreted dissolved ammonium was measured as a
function of body size (n = 460 individuals), and predictive models were
developed relating mass to excretion rates at the family level.
Mass-specific excretion rates ranged from 0.08 – 3.45 µmol·g-1·hr-1, and
per capita ammonium excretion rates ranged from 5.9 – 2765
µmol·individual·hr-1. Ammonium excretion scaled with fish body mass to the
¾ power, as predicted by the metabolic theory of ecology; mass-specific
excretion rates were higher in smaller fishes, but larger fishes
contributed more ammonium per individual. When controlling for body size,
ammonium excretion rates were greatest among surfperch (Embiotocidae),
damselfish (Pomacentridae), and wrasses (Labridae), and the general
trophic groups of planktivores and micro-carnivores. When body size
differences were considered, the greatest mean excretion rates per
individual were observed in larger-bodied species, such as California
Sheephead (Semicossyphus pulcher) and Lingcod (Ophiodon elongatus).
Empirical estimates of nutrient excretion by fishes, among the first
measured in temperate kelp forests, were consistent with those in other
aquatic systems. Ultimately, empirically derived excretion rates are the
first step to quantifying the relative importance of consumers to nutrient
cycling in kelp forest ecosystems.