Sponges can have powerful effects on ecosystem processes in shallow, tropical marine ecosystems and are an integral component of the bentho-pelagic cycle of nutrients, via filtering of dissolved and particulate organic matter from the water column. The diversity of marine communities is thought to play a determining role in intensity of ecosystem processes; thus the loss of taxa alters community function and by extension ecosystem processes. Coastal sponge populations worldwide are increasingly exposed to declining water quality that in several regions has resulted in mass sponge mortalities and reduced sponge diversity. In the Florida Keys (Florida, USA), for example, frequent cyanobacteria blooms have decimated coastal sponge communities. There were two objectives for this research. First, to experimentally establish the baseline effects of Florida Keys sponges, at ecologically relevant biomass levels, on various shallow water ecosystem processes and functions, and richness on water column properties. The results of this work demonstrated the importance of sponge biomass and species-specific filtration rates on the intensity of water column nutrient cycling, and its constituents. The second objective of this research was to develop an understanding of how sponges might interact in the wild, ultimately affecting the ecosystem processes and functions measured previously. The results of field manipulations, and sponge measurements plus water column sampling, conducted at multiple sites within Florida Bay showed clearly that the sponges of these back-water lagoons competed intensely for food, particularly in areas of higher biomass and slower water movement. Overall, this dissertation highlighted how reductions in the abundance and diversity of sponges in coastal ecosystems can drastically alter water column properties
