Seabirds can aggregate in large numbers on the structure provided by floating shellfish operations (hereafter farms), particularly during the fall/spring migrations. We have previously estimated bird-related nitrogen (N) fluxes on the scale of 0.2-16.9 g of nitrogen m-2 y-1 at several North Carolina farms, putting bird nutrient inputs on a similar scale as other major estuarine N fluxes. However, the biogeochemical effects of concentrating avian biomass—and consequently associated nutrient inputs—over these farms is unknown.
Given the relative insolubility of uric acid, the primary N component of guano, and its likely degradation in ammonium (NH4+) at the benthic-water interface, we sought to determine if water column [NH4+] at floating farms tracked with observed changes in bird biomass. Surface water samples were collected from the edge of five floating farms and paired, fixed-point reference locations biweekly during a daytime low tide from July to December (n=60; only first 40 shown). Samples were analyzed fluorometrically using the OPA method. The degree to which farm [NH4+] exceeded expected (i.e. reference) concentrations was positively correlated to observed bird biomass (Figure 1), suggesting that farm-scale biogeochemical hotspots may form from bird nutrient inputs. In environments where N is often limiting, the introduction of these nutrients may support local water column and benthic primary productivity and enrich food webs.
This work is a step towards understanding how the bird habitat created by floating farms can direct bird nutrients into farms and farm-adjacent ecosystems. It may be possible to place farms or farm-associated gear strategically to maximize benefits to birds and harness their nutrients in ways that (re-)create nutrient subsidies lost in many developed settings. Going forward, scientists and managers must work to determine if the potential ecological benefits of bird aggregations at floating farms can be balanced with the potential for shellfish contamination, as a best-case scenario may be one in which the ecological functions of these farms are maintained while human health risks are minimized.