Effects of short-term sediment nutrient enrichment and grazer <i>(Neritina reclivata)</i> removal on sediment microalgae in a shallow eutrophic estuary (Alabama, USA)

Abstract

The olive snail (Neritina reclivata) is ubiquitous in tropical and subtropical systems of the Gulf of Mexico, however its impacts on sediment microalgae have been little studied. Many coastal systems around the world are being eutrophied due to human activities, and seemingly they will continue to be eutrophied to a further extent in the future. Exploring the single and combined impacts of further nutrient enrichment and grazing by the olive snail on sediment microalgae in such eutrophic systems is an important question for our understanding and management of these systems. Here we examine the effects of short-term nutrient enrichment and grazing by the olive snail N. reclivata on sediment microalgal biomass and composition in a shallow eutrophic estuary (Weeks Bay, Alabama, USA) of the Northern Gulf of Mexico. For this, we performed a series of factorial experiments adding or not nutrients and removing or not the snail, for a total of four treatments in each experiment: ambient grazing, ambient nutrients; ambient grazing, increased nutrients; no grazing, ambient nutrients; and no grazing, increased nutrients. We did not find any significant impact of nutrient addition in any of the eight short-term (i.e. four days) experiments carried out. Impacts by the snail were minor; we only found a decrease in biomass due to snail grazing in one of the eight experiments, and no impacts on microalgal (i.e. diatom) composition. High ambient nutrient concentrations in the sediment porewater and low snail abundances on the sediment could explain these findings. Our results suggest that ephemeral, short-term nutrient pulses into eutrophic coastal systems of the Northern Gulf of Mexico, such as Weeks Bay (Alabama, USA), should not greatly affect the abundance of sediment microalgae, even though those pulses occur in well-lit areas. The results further suggest the snail N. reclivata is not a major control of sediment microalgal populations in the subtidal sedimentary areas studied. Our findings contrast with the results of past work in sediments with well-lit and nutrient poor conditions, or sediments with high densities of other snail grazers. In conjunction this and other investigations indicate that the response of sediment microalgae to nutrient enrichment and modified grazer abundance depends to a large extent on the initial levels of nutrient availability and grazing before the system is altered.

Effects of grazing and fertilization on epiphyte growth dynamics under moderately eutrophic conditions: implications for grazing rate estimates

“…The only instance of positive biomass response to nutrient enrichment found under the presence of grazers suggests that grazers may somewhat alleviate the restraining effects of thick epiphyte canopies and allow for significant increases of epiphyte biomass. These results are qualitatively similar to previous experiments that have shown little effect of nutrient enrichment on periphyton biomass under eutrophic conditions (Hillebrand & Kahlert 2002, 2012.…”

The Effect of Shading and Nutrient Addition on the Microphytobenthos, Macrofauna, and Biogeochemical Properties of Intertidal Flat Sediments

“…This was opposite to what would be predicted if algal growth was nutrient limited. Despite biotic processing of mangrove leaves increasing nutrients in sediment (Camilleri, 1992) and nutrient addition altering faunal assemblages (Morris and Keough, 2003); adding a source of nutrients to sediments does not always result in either measurable increases in nutrients (Douglas et al, 2016), nor changes to fauna (Rossi, 2006), nor an overall increase in primary productivity or biomass of microphytobenthos (Stutes et al, 2006;Cebrián et al, 2012), but can alter assemblage structure (Gladstone-Gallagher et al, 2014b), for example by inducing cyanobacterial blooms (Armitage and Fong, 2004).…”