Seagrass production around artificial reefs is resistant to human stressors

Abstract

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 ARs 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.

Keywords: carbon; excretion; fish; nitrogen; phosphorus; production.

Figure 1.

Map of research locations in (a) Abaco, The Bahamas—inset colours identify locations of (b) Man-O-War Cay (purple) and (c) Bight of Old Robinson (maroon). Map of research locations in (d) Haiti—inset colours identify locations of (e) Ile A Vache. AR in (f) Bight of Old Robinson and (g) Ile A Vache. Pink dots identify locations of ARs on (b) (MOW1–3), (c) (LH1–3) and (e) (H1–H8). White dot in (e) identifies AR that was destroyed (H7).

Figure 2.

Comparison among ARs and The Bahamas and Haiti. Histograms of fish total length in (a) The Bahamas (purple) and (b) Haiti (green)—each line indicates a given AR, filled distributions are for all ARs in each region. (c) Nitrogen (N) and (d) phosphorus (P) supply by fish communities on all ARs. Seagrass nutrients for (e) N and (f) P. In all cases, colours correspond to (a) and (b) for region. ‘**’ or ‘n.s.’ denote statistically significant differences (α = 0.05), or lack thereof, respectively, between regions. Sites within each island were ranked based on %P in ambient seagrass (Amb. SG) tissue (increasing darkness = increasing %P). Total phosphorus = TP; total nitrogen = TN.

Figure 3.

Comparison among ARs in The Bahamas (purple) and Haiti (green). Island-level means at 1 and 20 m are indicated by black and red dashed bars, respectively. ‘**’ or ‘n.s.’ denote statistical significance (α = 0.05), or lack thereof, respectively, between islands. Black and red values indicate differences at 1 m and 20 m, respectively. The darkness of the symbol indicates ARs in areas of greater human impacts for each respective region, based on %P in ambient seagrass. See electronic supplementary material, figure S3, for additional reef-level variables. SLA = specific leaf area; LAI = leaf area index.

Figure 4.

(a) Functional responses of seagrass traits and processes with respect to distance in Haiti (green) and The Bahamas (purple). Each line represents an individual AR corresponding to datapoints of the same colour. (b) Proportion of ARs that showed a significant nonlinear relationship with distance for each trait/process and collectively (overall). (c) Mean thresholds by which ARs within each region affect specific traits and processes. SLA = specific leaf area; LAI = leaf area index. (d) Effect size (slope of line between intercept and threshold) for traits and processes. ‘n.s.’ and ‘*’ indicate not significant and significant (α = 0.05), respectively. Comparisons were only made for trait/processes with sufficient significant effect—see electronic supplementary material, figure S4, for results from the remaining seagrass traits.

Figure 5.

Significant pathways linking the response variables and predictors in separate SEMs. Models are separated into the BGH (AR models (a,c)) and outside of the BGH (ambient models (b,d)) in The Bahamas (purple) and Haiti (green). Number associated with each arrow and arrow size indicate relative effect size.