The response of mudflat microphytobenthos to bottom-up and top-down factors in the Bay of Fundy

Abstract

Mudflats of the Bay of Fundy are habitats to a variety of organisms ranging from microbes to small invertebrates to mudsnails (Tritia obsoleta) and shorebirds. The main source of primary production in mudflats is microphytobenthos. Biotic and abiotic factors affect the abundance of microphytobenthos and ultimately the energy that most of the ecosystem relies on. The effect of bottom-up and top-down factors on mudflat ecosystems have been studied in the past. However, due to the complex overlapping trophic roles in the ecosystem, identifying factors that influence microphytobenthos abundance can be difficult. Previous studies have examined bottom-up and top-down factors, but have either investigated only top-down, or both bottom-up and top-down but without considering all predators known to be important. There is lack of research on the combined effect of known important bottom-up and top-down factors. We investigated the effect of nutrient availability and key predators on microphytobenthos abundance. A manipulative field experiment involving a combination of fertilizer addition and predator exclusion was performed in Grande Anse, in the Bay of Fundy in summer 2020. Fertilizer increased microphytobenthos abundance throughout the experiment, four weeks after application. A seasonal pattern in biofilm abundance was observed with a decrease in abundance form early July to late July and an increase from late July to late August. We found that mudsnails, which were present in low densities in this system, increased microphytobenthos abundance. We suggest bioturbation from mudsnails’ grazing stimulated nutrient recycling and increased microphytobenthos production. Mudsnails were more abundant in fertilized sites, as well as where birds could forage as compared to where birds were excluded. We propose that the increased presence of mudsnails where birds could forage could be attributed to bird droppings and surface disturbances, stimulating microphytobenthos production. Apart from mudsnails, invertebrate communities did not affect microphytobenthos abundance, but there were individual group responses to fertilizer and predation. In late August, Corophium volutator were more abundant in unfertilized, snail- and bird-excluded sites, and bivalves were less abundant in bird-included sites, regardless of fertilizer treatment. In late July, nematode abundance was higher in fertilized sites than unfertilized. These findings broaden our understanding of ecological factors that affect microphytobenthos abundance, and of mudflat ecology as whole.