Nyssa J. Silbiger

Coral reefs are among the world's most diverse ecosystems and contribute substantially to ocean productivity and the carbon budget. Disturbances are shifting reefs to altered community states, yet how these new communities indirectly alter organismal physiology by modifying the local biogeochemical environment remains largely unknown. We investigated this biogeochemical feedback in a 25‐d mesocosm experiment in Mo'orea, French Polynesia. We exposed the common coral Porites rus to the biogeochemical environments generated by three distinct communities representative of recent benthic states in this system: coral‐, macroalgae‐, and crustose coralline algae‐enriched relative to a seawater‐only control. While net community production was similar among communities, net community calcification was lowest in the macroalgae‐enriched state. Across treatments, community metabolism drove changes in the organic and inorganic carbon pool, with net community production increasing both pH and dissolved organic carbon. Using structural equation modeling, we tested how these community‐generated chemical legacies propagated through coral physiology. Elevated pH increased chlorophyll a content, which increased endosymbiont density, tissue biomass, gross photosynthesis, and respiration through a series of indirect pathways. Dissolved organic carbon had no detectable effect on downstream corals at 25 d. These results reveal a mechanistic pathway linking disturbance‐driven shifts in benthic communities to coral performance. Altered community metabolism modifies local carbon chemistry, which reshapes symbiont physiology and ultimately influences host metabolic function. By identifying the pathways through which community states engineer their chemical environment, this study provides a framework for predicting coral performance and ecosystem trajectories as reefs transition away from coral‐enriched states.

Macroalgae–herbivore interactions are fundamental to the stability of coastal ecosystems, and their vulnerability to climate-driven shifts will affect biodiversity and ecosystem functioning. However, the effects of climate change on species interactions may be context dependent and vary by seasonal oceanographic processes. We examined the effects of warming and acidification on an intertidal primary producer foundation species, Silvetia compressa, and an herbivore, Agathistoma eiseni, under differing upwelling regimes in early (low upwelling) and late spring (high upwelling). We measured metabolic rates including photosynthesis, respiration and calcification after 4 weeks of exposure to a range of nine pH levels (7.2–8.0) at two temperature levels (16°C, 20°C). Rockweed responded more to acidification than to warming, but showed different seasonal responses, decreasing photosynthetic rates in early spring and increasing rates during late spring. Their snail consumer, however, responded most strongly to temperature, increasing both respiration and calcification rates under warm conditions in late spring. Our findings demonstrate that seasonal differences can modulate responses of macroalgae and herbivores to changing environmental conditions. Warming may increase top-down effects through increased snail metabolism, whereas acidification affects rockweed production to affect bottom-up forcing. By assessing physiological responses across different ecological regimes, our study advances understanding of how top-down and bottom-up processes interact in intertidal ecosystems under global change.

Local-scale nutrient input from seabird guano and human wastewater are hypothesized to enhance and hinder coral performance, respectively, with seabird guano possibly mitigating heat stress. To test this hypothesis, we compared the effects of anthropogenic (wastewater effluent), seabird-derived (guano), and mineral nutrient (inorganic N and P) enrichment sources, in combination with a two-week heat-stress treatment, on the metabolism of two corals in Hawai’i: Montipora capitata and Porites compressa. At ambient temperatures, enrichment affected the species differently: P. compressa showed no response to enrichment, whereas M. capitata exhibited a 26–31% increase in primary production under guano enrichment, with no changes in respiration. Heating depressed M. capitata primary production by 48% and 66% in both control and inorganic-enriched treatments, respectively. However, no comparable thermal stress response was detected in effluent- and guano-enriched corals, suggesting that biotically-derived nutrients ameliorated thermal stress. Post-heating, M. capitata showed no residual effects of prior thermal stress: within each enrichment treatment, metabolic rates were similar between prior-temperature treatments. In contrast, P. compressa was highly sensitive to heat across all enrichment treatments, exhibiting 90-150% reductions in primary production and respiration that persisted weeks after heating ended (3-39% below ambient). Enrichment did not facilitate P. compressa thermal recovery and instead corresponded with dampened productivity relative to controls, both during and after heating. By directly comparing these enrichment sources in a controlled mesocosm study for the first time, this work reveals divergent coral responses that highlight the importance of distinguishing among nutrient sources when evaluating local drivers of coral resilience.

Benthic–pelagic coupling, the reciprocal exchange of materials between benthic and pelagic habitats, has traditionally emphasised pelagic influences on benthic systems. Yet, the role of benthic biological processes in shaping pelagic microbial dynamics remains underexplored. We investigated how surfgrass and mussels regulate nitrogen and dissolved organic matter (DOM) cycling and their cascading effects on heterotrophic bacteria in Oregon tide pools. We quantified biogeochemical fluxes and bacterial responses before and after foundation species removal during contrasting upwelling regimes. Mussel‐dominated pools released high concentrations of ammonium and nitrate, while surfgrass pools transformed DOM that fueled bacterial growth; upwelling intensified these benthic–pelagic linkages. Removing foundation species dampened nutrient release in mussel pools and reduced DOM‐fueled bacterial growth in surfgrass pools, ultimately decoupling benthic productivity from pelagic microbial growth. Our results demonstrate the critical role of foundation species to pelagic microbial processes and underscore the vulnerability of coastal microbial dynamics to their global decline.

Nutrient availability drives community structure and ecosystem processes, especially in tropical lagoons that are typically oligotrophic but often receive allochthonous inputs from land. Terrestrially derived nutrients are introduced to tropical lagoons by surface runoff and submarine groundwater discharge, which are influenced by seasonal precipitation. However, terrigenous inputs presumably diminish along the onshore–offshore gradients within lagoons. We characterized nutrient availability in the lagoons of a tropical high island, Moorea, French Polynesia, using spatially distributed measurements of nitrogen content in the tissues of a widespread macroalga during the rainy season over 4 yr. We used synoptic water column sampling to identify associations among macroalgal nutrient content and the composition of inorganic macronutrients, dissolved organic matter, and microbial communities. We paired these data with quantifications of land use in nearby watersheds to uncover links between terrestrial factors, aquatic chemistry, and microbial communities. Algal N content was highest near shore and near large, human‐impacted watersheds, and lower at offshore sites. Sites with high algal N had water columns with high nitrite + nitrate, silicate, and increased humic organic matter (based on a fluorescence Humification Index), especially following rain. Microbial communities were differentiated among nearshore habitats and covaried with algal N and water chemistry, supporting the hypothesis that terrigenous nutrient enrichment shapes microbial dynamics in otherwise oligotrophic tropical lagoons. This study reveals that land–sea connections create nutrient subsidies that are important for lagoon biogeochemistry and microbiology, indicating that future changes in land use or precipitation will modify ecosystem processes in tropical lagoons.

Nutrient availability drives community structure and ecosystem processes, especially in tropical lagoons that are typically oligotrophic but often receive allochthonous inputs from land. Terrestrially-derived nutrients are introduced to tropical lagoons by surface runoff and submarine groundwater discharge, which are influenced by seasonal precipitation. Lagoon habitats are distributed along an onshore-offshore gradient; terrigenous inputs presumably diminish along the same continuum. We characterized nutrient enrichment in the lagoons of a tropical high island, Moorea, French Polynesia, using spatially distributed measurements of nitrogen content in the tissues of a widespread macroalga during the rainy season over four years. We used synoptic water column sampling to identify associations among macroalgal nutrient content and the composition of inorganic macronutrients, dissolved organic matter, and microbial communities. We paired these data with quantifications of land use in nearby watersheds to uncover links between terrestrial factors, aquatic chemistry, and microbial communities. Algal N content was highest near shore and near large, human-impacted watersheds, and lower at offshore sites. Sites with high algal N had water columns with high nitrite + nitrate, silicate, and increased humic organic matter (based on a fluorescence humification index), especially following rain. Microbial communities were differentiated among nearshore habitats and covaried with algal N and water chemistry, supporting the hypothesis that terrigenous nutrient enrichment shapes microbial dynamics in otherwise oligotrophic tropical lagoons. This study reveals that land-sea connections create nutrient subsidies that are important for lagoon biogeochemistry and microbiology, indicating that changes in land use or precipitation will modify ecosystem processes in tropical lagoons.

The addition of terrestrial inputs to the ocean can have cascading impacts on coastal biogeochemistry by directly altering the water chemistry and indirectly changing ecosystem metabolism, which also influences water chemistry. Here, we use submarine groundwater discharge (SGD) as a model system to examine the direct geochemical and indirect biologically mediated effects of terrestrial nutrient subsidies on a fringing coral reef. We hypothesize that the addition of new solutes from SGD alters ecosystem metabolic processes including net ecosystem production and calcification, thereby changing the patterns of uptake and release of carbon by benthic organisms. SGD is a common land–sea connection that delivers terrestrially sourced nutrients, carbon dioxide, and organic matter to coastal ecosystems. Our research was conducted at two distinct coral reefs in Moʻorea, French Polynesia, characterized by contrasting flow regimes and SGD biogeochemistry. Using a Bayesian structural equation model, our research elucidates the direct geochemical and indirect biologically mediated effects of SGD on both dissolved organic and inorganic carbon pools. We reveal that SGD‐derived nutrients enhance both net ecosystem production and respiration. Furthermore, the study demonstrates that SGD‐induced alterations in net ecosystem production significantly influence pH dynamics, ultimately impacting net ecosystem calcification. Notably, the study underscores the context‐dependent nature of these cascading direct and indirect effects resulting from SGD, with flow conditions and the composition of the terrestrial inputs playing pivotal roles. Our research provides valuable insights into the interplay between terrestrial inputs and coral reef ecosystems, advancing our understanding of coastal carbon cycling and the broader implications of allochthonous inputs on ecosystem functioning.

Coral reefs are highly productive ecosystems where bacterioplankton play a critical role in recycling dissolved compounds, enabling corals to thrive in oligotrophic waters. Reefs worldwide exhibit trends of declining coral cover with concomitant increases in macroalgae. This project seeks to understand how these trends may alter microbial carbon and nutrient recycling supporting the resilience of reefs to climate change. Decadal data from the Moorea Coral Reef Long Term Ecological Research program (MCR LTER) show that reefs have suffered significant structural damages from past cyclonic events, ongoing anthropogenic impacts and predator outbreaks (Taramea or Crown-of-thorn starfish Acanthaster planci), resulting in phase shifts from coral to macroalgal dominance. Previous research suggests that phase shifts reduce microbial diversity, increase heterotrophic bacteria and disrupt bacterioplankton recycling, leading to inefficient productivity and further coral decline. Mesocosm experiments simulated coral-dominated, algae-dominated, and post-disturbance reefs (A. planci outbreak) to assess the effects of benthic cover on compositional dynamics of microbial communities and dissolved organic matter. Organic matter (fDOM and DOC) quantification was done via fluorescence spectroscopy and catalytic oxidation, while microbial community analysis was done using flow cytometry with amplicon next generation DNA sequencing. The results of this project will enhance our understanding of the interactions between bacterioplankton communities and benthic species, with implications for carbon fixation and remineralization processes in the oligotrophic ecosystems of the South Pacific. Additionally, findings from this study will provide a valuable baseline for assessing the ecological status of locally managed lagoons, supporting parallel research efforts in remote regions of Tahiti.

Coral reefs experience numerous natural and anthropogenic environmental gradients that alter biophysical conditions and affect biodiversity. While many studies have focused on drivers of reef biodiversity using traditional diversity metrics (e.g., species richness, diversity, evenness), less is known about how environmental variability may influence functional diversity. In this study, we tested the impact of submarine groundwater discharge (SGD) on taxonomic and functional diversity metrics in Mo‘orea, French Polynesia. SGD is the expulsion of terrestrial fresh or recirculated seawater into marine environments and is associated with reduced temperatures, pH, and salinity and elevated nutrient levels. Using a regression approach along the SGD gradient, we found that taxon and functional-entity richness displayed unimodal relationships to SGD parameters, primarily nitrate + nitrite and phosphate variability, with peak richness at moderate SGD for stony coral and the full benthic community. Macroalgae showed this unimodal pattern for functional-entity but not taxonomic richness. Functional community composition (presence and abundance of functional entities) increased along the gradient, while taxonomic composition showed a nonlinear relationship to SGD-related parameters. SGD is a common feature of many coastal ecosystems globally and therefore may be more important to structuring benthic functional diversity than previously thought. Further, studying community shifts through a functional-trait lens may provide important insights into the roles of community functions on ecosystem processes and stability, leading to improved management strategies.