DOI: 10.5061/dryad.37pvmcvqn
title: Seagrass production around artificial reefs is resistant to human stressors

description: 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.
description: 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
 <sup>13</sup>C and <sup>15</sup>N. 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.