The effects of top-down and bottom-up forces on the nutritional quality at an intertidal mudflat in the Bay of Fundy

Abstract

Mudflats in the upper Bay of Fundy are highly productive ecosystems that support shorebird and invertebrate life. The base of the food web in these mudflats is a biofilm containing microphytobenthos— photosynthetic microorganisms that are the main primary producers in mudflat ecosystems. Major mudflat consumers, like snails and shorebirds, can interact with biofilm in complex ways through predation on biofilm consumers as well as direct consumption and physical disturbance of the biofilm. The goal of this study was to determine the ecological factors that affect the nutritional quality (measured as content of protein, carbohydrate, and total organic matter) of mudflat biofilm. The nutritional content of biofilm was measured during an experimental manipulation of the top-down effects of key predators (mud snails and Calidrid shorebirds) and the bottom-up effects of increased nutrient availability for microphytobenthos from July – Aug, the annual period when mudflat consumers are most active and abundant. We established a repeated measures split-plot experiment at Grande Anse, in the upper Bay of Fundy, in summer of 2020 in which control plots, bird exclosures, and bird and snail (full) exclosures were established at each of twenty sites with ten of these sites randomly selected to be fertilized. We collected biofilm samples from all treatments in all sites as sediment contact cores of the top 2 mm of sediment during three sampling periods: 3-4 July, 30-31 July, and 27-28 August. We found clear temporal trends in biofilm protein and carbohydrate content, with the absolute abundance of both declining after the first sampling period and a corresponding increase in organic matter-normalized protein content. Predator treatments also had a significant effect on the absolute carbohydrate content in late August in unfertilized plots, with more carbohydrate in bird exclosures compared to controls. Fertilizer treatment significantly decreased the organic matter-normalized carbohydrate content in late July. These findings indicate that biofilm protein and carbohydrate content are affected by seasonal factors and that their abundance might not be limited by external nutrient supply (or at least it does not increase with nutrient addition). Additionally, this work shows that predators, such as shorebirds, might alter the nutritional content of mudflat biofilm in addition to being supported by it.