Anthropogenic versus fish‐derived nutrient effects on seagrass community structure and function

Abstract

Humans are altering nutrient dynamics through myriad pathways globally. Concurrent with the addition of nutrients via municipal, industrial, and agricultural sources, widespread consumer exploitation is changing consumer-mediated nutrient dynamics drastically. Thus, altered nutrient dynamics can occur through changes in the supply of multiple nutrients, as well as through changes in the sources of these nutrients. Seagrass ecosystems are heavily impacted by human activities, with highly altered nutrient dynamics from multiple causes. We simulate scenarios of altered nutrient supply and ratios, nitrogen:phosphorus (N:P), from two nutrient sources in seagrass ecosystems: anthropogenic fertilizer and fish excretion. In doing so we tested expectations rooted in ecological theory that suggest the importance of resource dynamics for predicting primary producer dynamics. Ecosystem functions were strongly altered by artificial fertilizer (e.g., seagrass growth increased by as much as 140%), whereas plant/algae community structure was most affected by fish-mediated nutrients or the interaction of both treatments (e.g., evenness increased by ~140% under conditions of low fish nutrients and high anthropogenic nutrients). Interactions between the nutrient sources were found for only two of six response variables, and the ratio of nutrient supply was the best predictor for only one response. These findings show that seagrass structure and function are well predicted by supply of a single nutrient (either N or P). Importantly, no single nutrient best explained the majority of responses-measures of community structure were best explained by the primary limiting nutrient to this system (P), whereas measures of growth and density of the dominant producer in the system were best explained by N. Thus, while our findings support aspects of theoretical expectations, the complexity of producer community responses belies broad generalities, underscoring the need to manage for multiple simultaneous nutrients in these imperiled coastal ecosystems.

Rewiring coral: Anthropogenic nutrients shift diverse coral–symbiont nutrient and carbon interactions toward symbiotic algal dominance

“…Fish‐mediated nutrient supply was altered by reducing the physical complexity of the reefs by filling in the holes of the cinder blocks creating a smooth‐sided structure. In doing so, fish biomass and community composition were substantially altered, but in a continuous manner, across reefs, that is, even the half of the reefs with low physical structure had varying fish biomass (Allgeier et al., 2018; Figure 1a; Appendix S1). Reefs were placed >150 m apart to minimize among‐reef movement of more transient fish species (Carr & Hixon, 1995, 1997).…”

“…We took advantage of an ongoing, artificial reef nutrient enrichment study, see Allgeier et al. (2018). The study included 16 artificial reefs, constructed in December 2010 from 30 cinder blocks (~40 cm × 20 cm × 20 cm) in a pyramid shape (~100 cm × 80 cm at base, 60 cm height), on sparse seagrass habitat dominated by common turtle grass, Thalassia testudinum , at a depth of 3–4 m. Artificial reefs provide replicable units of discrete size from which ecological responses in the local ecosystem can be measured (Carr & Hixon, 1997; Hixon & Beets, 1989).…”

“…Fish‐mediated nutrient supply was quantified by modeling species‐specific nutrient supply rates onto repeated visual census data that estimated fish abundance and size. This approach has been conducted previously by the authors on these same reefs (see Allgeier et al., 2018), other artificial reefs nearby (Allgeier, Yeager, & Layman, 2013; Layman et al., 2013), and other coral reef ecosystems in the Caribbean (e.g., Allgeier, Layman, Mumby, & Rosemond, 2015), and is described in greater detail in Appendix S1. Importantly, because the exact density of fish could not be explicitly manipulated, the high/low fish treatment also provided a continuous gradient of fish‐mediated nutrient supply across all 16 reefs used for Objectives 2 and 3 (Figure 1a).…”

“…Reefs were placed >150 m apart to minimize among-reef movement of more transient fish species (Carr & Hixon, 1995, 1997. Cross-reef enrichment due to fertilizer was not a concern due to the highly oligotrophic nature of the system with high rates of nutrient uptake by benthic and water column producers, and previous evidence in this system that nutrient effects do not extend beyond ~8 m away from reefs (Allgeier et al, 2018;Layman, Allgeier, & Montaña, 2016;Layman, Allgeier, Yeager, & Stoner, 2013). These conditions are consistent with previous findings on the effects of nutrient enrichment on water column nutrients from an oligotrophic embayment in Hawaii (Smith, Kimmerer, Laws, Brock, & Walsh, 1981).…”

“…The study was conducted in The Bight of Old Robinson, Abaco, The Bahamas (26.26697, -76.94126), a semi-enclosed bay dominated by seagrass that is interspersed with sand and hard bottom habitats (Yeager, Allgeier, & Layman, 2011). We took advantage of an ongoing, artificial reef nutrient enrichment study, see Allgeier et al (2018).…”