DOI: 10.5061/dryad.0p2ngf29v
title: Empirical measurements of ammonium excretion in kelp forest fishes: Effects of body size, taxonomy and trophic guild

description: 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.
description: To quantify rates of ammonium excretion for the dominant fish
 species inhabiting nearshore rocky reefs and kelp beds in central and
 southern California, we utilized established techniques in the field for
 rapid empirical estimation of consumer-derived nutrient excretion in
 aquatic systems. We selected a subset of 30 species, from 13 families, to
 represent the dominant fish species comprising California kelp forests.
 The selected species represented ~85% of total fish biomass, based on
 estimates of density and biomass from extensive underwater visual surveys
 conducted by the Partnership for Interdisciplinary Studies of Coastal
 Oceans (PISCO) in central and southern California. Each fish species was
 assigned to one of five trophic guilds based on discrete, fixed trophic
 delineations (herbivore/omnivore, planktivore, micro-carnivore,
 macro-carnivore, and piscivore) as established in the literature.
 Herbivores/omnivores consist of species that consume a relatively large
 proportion of algae along with some animal matter, planktivores eat
 zooplankton, micro-carnivores eat small crustaceans (e.g., amphipods,
 isopods, shrimps, crabs, and copepods), macro-carnivores eat larger
 invertebrates (e.g., gastropods, crabs, polychaetes, shrimps, sea urchins)
 and may occasionally eat small fishes, and piscivores eat primarily
 fishes. Since many fishes within California kelp forests are opportunistic
 predators, species with a mixed diet were classified into a single trophic
 group based on the dominant type of prey taxa (&gt;50% of the diet)
 consumed at the adult stage. Fish collections occurred
 from May 2016 – September 2017. To quantify excretion rates for species
 representative of kelp forests along the entire coast of California, we
 sampled fish in both central and southern California. Species affiliated
 with colder water were collected from Stillwater Cove
 (36°33'42.8" N 121°56'48.5" W) and Point Lobos
 (36°31'18.4" N 121°56'33.9" W) in central California,
 whereas species affiliated with warmer water were collected off Santa
 Catalina Island (33°26'52.8" N 118°28'57.4 "W) in
 southern California. For each species, we collected
 <em>n </em>= 5-45 individuals across a range of body sizes via
 a variety of methods, including hook-and-line and baited fish traps, as
 well as barrier nets and hand nets while SCUBA diving. Species with fewer
 than 5 individuals collected were pooled with similarly related species in
 the same family during the initial stage of species-level data analysis.
 Fish were collected during daylight hours between 0900 and 1500, and only
 individuals collected in good condition (i.e., not exhibiting signs of
 barotrauma) underwent experimental incubation to measure ammonium
 excretion rates. After capture, fish were either placed directly in a
 prepared incubation chamber or in a communal holding tank for &lt;30
 minutes, in cases where the fishing yield exceeded the rate of
 experimental processing. All containers with fish were covered and aerated
 to reduce stress. To reduce handling stress, we weighed each individual
 for wet mass (in grams) and measured total and standard length (in
 centimeters) following incubation. Fish were then released alive at the
 site of capture. To calculate the ammonium excretion
 rate (NH<sub>4</sub><sup>+</sup> per individual
 per gram body weight per hour) for each individual fish, we conducted
 experimental incubations to test the change in ammonium over a set time
 period. Each tank was lined with a 4 mm polyethylene bag, and the bag was
 changed between incubations. The tanks were filled with a known volume of
 seawater (1-75 L) which varied according to the size of the fish. During
 data analysis, nutrient excretion rates were calculated by standardizing
 the measured nutrient concentration by the water volume used. Control
 incubations without fish were conducted (n=4 per day) to quantify any
 changes in ammonium levels not due to excretion by the fish.
 Samples for ammonium quantification underwent nutrient analysis
 on the day of collection using established fluorometric techniques. In the
 field, we added seawater samples to amber vials containing working
 reagents to begin the reaction process. The working reagent contained a
 mix of ortho-phthalaldehyde (OPA), sodium sulfite, and sodium tetraborate.
 Within 3-10 hours of collection, we analyzed the ammonium samples using a
 handheld Turner Designs AquaFluor Handheld Fluorometer (optical kit No.
 8000-402). Standards with known concentrations of ammonium were used daily
 to create a standard curve for calculating ammonium concentrations and
 underwent the same treatment in the field. To calculate
 the excretion rate of each individual, we plotted
 NH<sub>4</sub><sup>+</sup> (µmol) as a function of
 experimental duration (minutes) and extracted the slope of the
 relationship using linear regression. Mass-specific excretion rates by
 species were calculated as total ammonium excreted per individual per
 hour, divided by the wet weight (g) of an individual. Per capita excretion
 rates were averaged over all sizes measured for that species, and reflect
 the total ammonium excreted, regardless of body size. Species with similar
 taxonomic identities and/or feeding habits were later grouped together and
 excretion was calculated at the level of a fish family or trophic guild,
 respectively. To account for the effect of fish mass on
 excretion rates at the species-, family-, and trophic group levels, we
 used bivariate linear regressions to examine the strength of the
 association between NH<sub>4</sub><sup>+</sup>
 excretion per individual per minute as a function of fish mass. Due to
 evidence of allometric scaling at the larger fish weights, we
 log-transformed excretion and body size to improve homoscedasticity and
 normality. To test the hypothesis that the concentration of ammonium
 excreted by kelp forest fishes differs based on phylogeny, we used an
 Analysis of Covariance (ANCOVA) with the factor of family identity, the
 covariate of body mass, and the interaction between family identity and
 body mass. The families Cottidae, Gobiidae, and Malacanthidae were
 excluded due to limited sample sizes and/or a limited range of body sizes
 sampled. To test the hypothesis that the concentration of ammonium
 excreted by kelp forest fishes differs among trophic guilds, we used an
 ANCOVA to test for statistically significant differences in mass-specific
 excretion rates with the factor of trophic guilds, the covariate of body
 size, and the interaction between the two.
description: # Empirical measurements of ammonium excretion in kelp forest fishes:
 Effects of body size, taxonomy and trophic guild
 [https://doi.org/10.5061/dryad.0p2ngf29v](https://doi.org/10.5061/dryad.0p2ngf29v) ## Description of the data and file structure The data consists of experimental incubations of kelp forest fish in California, USA across 30 species and a range of body sizes, measuring ammonium excretion rates. ### Files and variables #### File: Shrestha\_kelp\_forest\_fish\_ammonium\_excretion\_all\_fish.xlsx **Description:** This data set contains measurements of ammonium excretion rates of kelp forest fishes from central and southern California. This data set contains information for every individual and species incubated to measure ammonium excretion ##### Variables * Fish_ID - Specific ID number for each fish in the data set * Region - Region in California where fish were collected (central or southern California) * Collection location-specific site where the fish were collected * Catch method - the technique used to catch the fish for ammonium excretion measurements * SP_CODE - Four letter species code, consisting of the first letter of genus and the first three letters of the species name * WM_g - The weight mass of the fish measured in grams * exc_rate_NH4_umol_min-1 - The estimated ammonium excretion rate in micro mols per min * exc_rate_NH4_umol_hour-1 - The estimated ammonium excretion rate in micro mols per hour * Mass specific excretion (NH4 per g per hour) - Mass specific excretion per gram of fish * Size_cm - The measured total length of the fish in centimeters * TAXON_GENUS - The genus of the fish * TAXON_SPECIES - The species of the fish * TAXON_ORDER - The order of the fish * TAXON_FAMILY - The family of the fish * Functional group - The functional/trophic group of each fish based on the literature #### File: Shrestha\_kelp\_forest\_fish\_ammonium\_excretion\_subset.xlsx **Description:** This data set includes a subset of the ammonium excretion data for commonly captured fish species used to generate linear regression relationships predicting excretion rate as a function of body mass at the family level. ##### Variables * Fish_ID - Specific ID number for each fish in the data set * FAMILY - The family of the fish * SPECIES_CATEGORY - The genus and species of each fish * EXCR (umol indiv-1 hr-1) - ammonium excretion estimated in micromol per individual per hour * LOG 10 MASS (g) - Log of the wet mass of the fish * LOG 10 EXCR - Log of the ammonium excretion rate of each fish