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A fundamental goal in ecology is to understand the role of consumers in top-down (TD) and bottom-up (BU) processes that affect the functioning of ecosystems. Consumers ingest organic matter and excrete inorganic nutrients, and individual roles are influenced by body size and functional identity. Our study quantifies how alterations to herbivore assemblages affect both TD and BU processes on coral reefs in the South Pacific. We collected empirical data on consumption and nutrient excretion rates from 300 individual herbivorous fishes belonging to five functional groups. Individual-level traits were then scaled to a 13-year time series of fish populations from reefs that have either shifted to algal dominance or remained in the coral state. Large excavating parrotfishes and other herbivores on reefs in the coral state contributed 43% more herbivory and excreted nutrients at a higher ratio of N:P than herbivores on algal-dominated reefs; both processes may benefit coral health. Algal-dominated reefs experienced 56% higher rates of detritivory by large detritivorous fishes that remove detritus from algal surfaces, a process that may facilitate algal dominance. By scaling individual-level traits to population time series, our study provides a framework to quantify how changes to consumer assemblages impact both TD and BU processes across ecosystems undergoing change. Identifying the unique roles of consumers in processes that maintain and reinforce ecosystem states is the key to predicting the importance of shifts in diverse consumer assemblages. [ABSTRACT FROM AUTHOR]

Consumers mediate nutrient cycling through excretion and egestion across most ecosystems. In nutrient‐poor tropical waters such as coral reefs, nutrient cycling is critical for maintaining productivity. While the cycling of fish‐derived inorganic nutrients via excretion has been extensively investigated, the role of egestion for nutrient cycling has remained poorly explored. We sampled the fecal contents of 570 individual fishes across 40 species, representing six dominant trophic guilds of coral reef fishes in Moorea, French Polynesia. We measured fecal macro‐ (proteins, carbohydrates, lipids) and micro‐ (calcium, copper, iron, magnesium, manganese, zinc) nutrients and compared the fecal nutrient quantity and quality across trophic guilds, taxa, and body size. Macro‐ and micronutrient concentrations in fish feces varied markedly across species. Genera and trophic guild best predicted fecal nutrient concentrations. In addition, nutrient composition in feces was unique among species within both trophic guilds (herbivores and corallivores) and genera (Acanthurus and Chaetodon). Particularly, certain coral reef fishes (e.g., Thalassoma hardwicke, Chromis xanthura, Chaetodon pelewensis and Acanthurus pyroferus) harbored relatively high concentrations of micronutrients (e.g., Mn, Mg, Zn and Fe, respectively) that are known to contribute to ocean productivity and positively impact coral physiological performances. Given the nutrient‐rich profiles across reef fish feces, conserving holistic reef fish communities ensures the availability of nutritional pools on coral reefs. We therefore suggest that better integration of consumer egestion dynamics into food web models and ecosystem‐scale processes will facilitate an improved understanding of coral reef functioning. [ABSTRACT FROM AUTHOR]

The supply of nutrients from consumer waste products, i.e., consumer-mediated nutrient dynamics, is critical for primary production in marine environments, especially in oligotrophic systems. However, human impacts can influence the effects of consumer-mediated nutrients in two main ways: (1) overfishing can reduce consumer populations and thus the nutrients they supply, (2) anthropogenic nutrient enrichment can shift baselines of nutrient limitation. Previous work has shown that artificial reefs create biogeochemical hot spots from which nutrient responses by surrounding primary producers can be measured. We constructed eight artificial reefs from 88 cinder blocks in shallow seagrass beds near Ile-a-Vache, Haiti, to test for interacting effects of fish-mediated nutrients and anthropogenic nutrients on growth rates of the dominant seagrass, Thalassia testudinum, surrounding the artificial reefs. We found that (1) anthropogenic nutrient enrichment appears to be the primary factor influencing seagrass growth close to the artificial reef, and (2) despite overall anthropogenic nutrient enrichment effects, consumer-mediated nutrients still generate biogeochemical hot spots and the two nutrient sources additively influence seagrass growth rates. We also found that nitrogen and phosphorus similarly predict seagrass growth rates, and better so than their ratio, indicating the supply rates of nitrogen and phosphorus are both limiting seagrass production. Results from this study indicate that the effect of anthropogenic nutrients can strongly influence seagrass growth in these systems, but consumers can supply a source of nutrients that has additive effects on seagrass growth. [ABSTRACT FROM AUTHOR]

Energy flow and nutrient cycling dictate the functional role of organisms in ecosystems. Fishes are key vectors of carbon (C), nitrogen (N) and phosphorus (P) in aquatic systems, and the quantification of elemental fluxes is often achieved by coupling bioenergetics and stoichiometry. While nutrient limitation has been accounted for in several stoichiometric models, there is no current implementation that permits its incorporation into a bioenergetics approach to predict ingestion rates. This may lead to biased estimates of elemental fluxes.Here, we introduce a theoretical framework that combines stoichiometry and bioenergetics with explicit consideration of elemental limitations. We examine varying elemental limitations across different trophic groups and life stages through a case study of three trophically distinct reef fishes. Further, we empirically validate our model using an independent database of measured excretion rates.Our model adequately predicts elemental fluxes in the examined species and reveals species‐ and size‐specific limitations of C, N and P. In line with theoretical predictions, we demonstrate that the herbivore Zebrasoma scopas is limited by N and P, and all three fish species are limited by P in early life stages. Further, we show that failing to account for nutrient limitation can result in a greater than twofold underestimation of ingestion rates, which leads to severely biased excretion rates.Our model improved predictions of ingestion, excretion and egestion rates across all life stages, especially for fishes with diets low in N and/or P. Due to its broad applicability, its reliance on many parameters that are well‐defined and widely accessible, and its straightforward implementation via the accompanying r‐package fishflux, our model provides a user‐friendly path towards a better understanding of ecosystem‐wide nutrient cycling in the aquatic biome. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Nitrogen pollution increases the susceptibility of corals to heat-induced bleaching. However, different forms of nitrogen (nitrate vs. ammonium/urea) may have different impacts on thermal tolerance of corals. We used an 18-month field experiment on the oligotrophic fore reef of Moorea, French Polynesia, to test how different forms of nitrogen (nitrate vs. urea) impacted coral bleaching. The experiment spanned two moderate thermal stress events in 2016 and 2017. Nitrate increased bleaching prevalence in Acropora by up to 100% and in Pocillopora by up to 60% compared to control corals. Urea exposure often had intermediate effects on bleaching (not different from either control or nitrate-exposed corals) in both taxa. Importantly, nitrate prolonged bleaching in both Acropora and Pocillopora as nitrate-exposed corals remained bleached even after thermal stress ended, while control and urea-exposed corals had mostly recovered. Nitrate exposure also increased the prevalence of partial mortality in Pocillopora colonies and more than tripled the number of colonies that completely died. Our data are the first to show contrasting effects of different forms of nitrogen on coral bleaching and mortality in a natural reef environment, linking previous patterns from large-scale correlative studies with results from more mechanistic laboratory experiments. Most importantly, we showed that corals exposed to nitrate exhibited more frequent bleaching, bleached for longer duration, and were more likely to die than corals in low nitrogen conditions. Exposure to excess nitrogen, particularly anthropogenic nitrogen, may lower the temperature threshold at which corals bleach, triggering bleaching events on polluted reefs even when typical thermal stress thresholds have not been crossed. [ABSTRACT FROM AUTHOR]

Climate change is increasing the frequency and magnitude of temperature anomalies that cause coral bleaching, leading to widespread mortality of stony corals that can fundamentally alter reef structure and function. However, bleaching often is spatially variable for a given heat stress event, and drivers of this heterogeneity are not well resolved. While small-scale experiments have shown that excess nitrogen can increase the susceptibility of a coral colony to bleaching, we lack evidence that heterogeneity in nitrogen pollution can shape spatial patterns of coral bleaching across a seascape. Using island-wide surveys of coral bleaching and nitrogen availability within a Bayesian hierarchical modeling framework, we tested the hypothesis that excess nitrogen interacts with temperature anomalies to alter coral bleaching for the two dominant genera of branching corals in Moorea, French Polynesia. For both coral genera, Pocillopora and Acropora, heat stress primarily drove bleaching prevalence (i.e., the proportion of colonies on a reef that bleached). In contrast, the severity of bleaching (i.e., the proportion of an individual colony that bleached) was positively associated with both heat stress and nitrogen availability for both genera. Importantly, nitrogen interacted with heat stress to increase bleaching severity up to twofold when nitrogen was high and heat stress was relatively low. Our finding that excess nitrogen can trigger severe bleaching even under relatively low heat stress implies that mitigating nutrient pollution may enhance the resilience of coral communities in the face of mounting stresses from global climate change. [ABSTRACT FROM AUTHOR]

The transformation of coral reefs has profound implications for millions of people. However, the interactive effects of changing reefs and fishing remain poorly resolved. We combine underwater surveys (271 000 fishes), catch data (18 000 fishes), and household surveys (351 households) to evaluate how reef fishes and fishers in Moorea, French Polynesia responded to a landscape-scale loss of coral caused by sequential disturbances (a crown-of-thorns sea star outbreak followed by a category 4 cyclone). Although local communities were aware of the disturbances, less than 20% of households reported altering what fishes they caught or ate. This contrasts with substantial changes in the taxonomic composition in the catch data that mirrored changes in fish communities observed on the reef. Our findings highlight that resource users and scientists may have very different interpretations of what constitutes 'change' in these highly dynamic social–ecological systems, with broad implications for successful co-management of coral reef fisheries. [ABSTRACT FROM AUTHOR]