Waterbird zoogeochemical effects shape wetland nutrient dynamics and algal communities in Laurentian Great Lakes coastal freshwater wetlands

Freshwater wetlands along the Laurentian Great Lakes provide vital ecosystem services, two of which include providing habitat for millions of waterfowl and reducing nutrient (nitrogen (N) and phosphorus (P)) runoff from the landscape. Nutrient bioavailability in wetlands is influenced by waterbirds, which shape biogeochemical dynamics through the excretion of N- and P-rich guano. There has been limited focus on how waterbirds shape nutrient budgets (i.e. via zoogeochemical effects) in wetlands that are managed for both waterfowl habitat and nutrient retention. Therefore, we examined how three distinct functional groups of common waterbirds influence nutrient biogeochemistry and algal community structure in two coastal freshwater wetlands along Lake Erie (Ohio, USA). We experimentally measured how waterbird-derived nutrient contributions shape wetland nutrient removal and elicit changes in algal community composition through experimental additions of guano to microcosms. We collected biweekly waterbird densities throughout 1 year and used guano nutrient content and dropping rates to estimate annual waterbird N and P fluxes. We then assessed the relative importance of waterbird-nutrient contributions through comparisons to previously monitored net nutrient loads from landscape runoff. Waterbird guano additions initially increased nutrient concentrations in microcosms; however, wetland sediments rapidly removed most of the total P, but only approximately half of the total N from the water column. Waterbird guano from different functional groups favoured distinct algal communities, driven primarily by differences in guano total N content. Waterbird-derived nutrient fluxes were highest during fall migration and may be locally important in wetlands during nutrient-limiting times. However, annual waterbird-derived nutrient fluxes are often negligible (<1.3%) compared with external loads from surrounding agriculture-dominated watersheds. By understanding the zoogeochemical effects that waterbirds have in freshwaters, we can inform management decisions that simultaneously protect waterfowl habitat and enhance nutrient removal processes to maximize multiple ecosystem services in the Great Lakes region.