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Anthropogenic stressors pose substantial threats to the existence of coral reefs. Achieving successful coral recruitment stands as a bottleneck in reef restoration and hybrid reef engineering efforts. Here, we enhance coral settlement through the development of biomimetic microhabitats that replicate the chemical landscape of healthy reefs. We engineered a soft biomaterial, SNAP-X, comprising silica nanoparticles (NPs), biopolymers, and algal exometabolites, to enrich reef microhabitats with bioactive molecules from crustose coralline algae (CCA). Coral settlement was enhanced over 20-fold using SNAP-X-coated substrates compared with uncoated controls. SNAP-X is designed to release chemical signals slowly (>1 month) under natural seawater conditions, and can be rapidly applied to natural reef substrates via photopolymerization, facilitating the light-assisted 3D printing of microengineered habitats. We anticipate that these biomimetic chemical microhabitats will be widely used to augment coral settlement on degraded reefs and to support ecosystem processes on hybrid reefs. 

Abstract Marine organisms are increasingly recognized as both responding to and driving biogeochemical changes in their environment. The addition of exogenous resources to the ocean, such as nutrients, that alter organismal physiology can lead to biogeochemical cascades wherein these solutes both alter water chemistry directly and indirectly by changing biological processes that influence water chemistry. To quantify how allochthonous nutrients drive biogeochemical cascades, we measured a suite of biogeochemical parameters during synoptic spatial surveys across two reefs in Mo’orea, French Polynesia conducted day and night at both low and high tide in two different seasons. These data were used to build a model that demonstrates how inputs of nutrients to coral reefs via submarine groundwater discharge directly alter reef metabolism with cascading effects on the cycling of dissolved organic and inorganic carbon that regulate productivity, calcification, and the microbial loop. 

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. 

Abstract 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.