Primary production underpins most ecosystem services, including carbon sequestration and fisheries. Artificial reefs (ARs) are widely used for fisheries management. Research has shown that a mechanism by which AR in seagrass beds can support fisheries and carbon sequestration is through increasing primary production via fertilization from aggregating fish excretion. Seagrass beds are heavily affected by anthropogenic nutrient input and fishing that reduces nutrient input by consumers. The effect of these stressors is difficult to predict because impacts of simultaneous stressors are typically non-additive. We used a long-term experiment to identify the mechanisms by which simultaneous impacts of sewage enrichment and fishing alter seagrass production around ARs across non-orthogonal gradients in human-dominated and relatively unimpacted regions in Haiti and The Bahamas. Merging trait-based measures of seagrass and seagrass ecosystem processes, we found that ARs consistently enhanced per capita seagrass production and maintained ecosystem-scale production despite drastic shifts in controls on production from human stressors. Importantly, we also show that coupled human stressors on seagrass production around ARs were additive, contrasting expectations. These findings are encouraging for conservation because they indicate that seagrass ecosystems are highly resistant to coupled human stressors and that ARs promote ecosystem services even in human-dominated ecosystems.

The study aimed to examine the effect of fishing pressure and anthropogenic nutrient pollution on the nutrient supply in artificial reefs (ARs).The study involved the construction and deployment of ARs in seagrass habitats dominated by turtle grass in two locations in The Bahamas and one in Haiti in 2014. The ARs were constructed from cinder blocks and were deployed at similar depths. 

In May and June 2018, measurements were taken at two levels - at the AR level and on a plot level (within each AR) - in order to quantify seagrass and sediment nutrient content, fish biomass and nutrient supply, and seagrass variables. AR level measurements were taken 1 meter from the AR and plot-level measurements were taken using three transects oriented ~120° apart with eight distances (0.5, 1, 2, 3, 4, 6, 12, and 20 m from the AR) resulting in 24 sampling “plots” per AR.

Samples of seagrass and sediment were processed at the Department of Ecology and Evolutionary Biology, University of Michigan, USA. The seagrass shoots were measured for growth, length, and width, and the number of visible bites taken from each shoot was counted to estimate herbivory. Seagrass and sediment samples were lyophilized and analyzed for elemental content of carbon and nitrogen, and the natural abundance of stable isotopes ¹³C and ¹⁵N. Phosphorus was quantified using dry oxidation acid hydrolysis extraction followed by colorimetric analysis, and water column nitrate and phosphate were analyzed using continuous flow analysis. Total P and total N were analyzed using an acid and alkaline persulfate digestion, respectively, prior to continuous flow analysis.

To estimate fish abundance and nutrient supply rates in the study, underwater visual censuses were conducted on each AR, and excretion models were used to estimate fish nutrient supply rates. The models were generated using a Bayesian framework.