Sponges are a common component of seagrass ecosystems throughout the Tropical Atlantic and Caribbean (see a previous post about seagrass-sponge interactions here). We know that sponges filter a large amount of water and in the process provide a link between the water column and the benthic community. We also know that sponges (and the microbial community that lives in them) are capable of completing all the steps of the nitrogen cycle and act as large sources of dissolved inorganic nitrogen (ammonia, nitrate, and nitrite – the types of nitrogen plants like seagrass need to grow). Many animals make their homes in sponges and other animals eat sponges. What we do not know is how sponges may shape seagrass ecosystems. This week I finished preliminary sampling on a long term experiment designed to assess how the presence of a sponge (Ircinia felix) changes seagrass ecosystem structure and function.
We know that primary production in Bahamian waters is largely limited by the availability of nitrogen and phosphorus (see previous posts by Jake Allgeier here). Therefore, a consistent source of these limiting nutrients can increase rates of seagrass and macro algal growth. Ircinia felix, commonly known as stink sponge, is a nitrifying sponge. This means that it converts nitrogen into nitrate or nitrite, which is a form plants can use. We know very little about how sponges process phosphorus, but the small amount of work that has been done suggests that sponges are sources of phosphorus as well. The experiment I set up this week is designed to measure the response of a seagrass bed to the presence of Ircinia felix. After several weeks of quantifying the ecosystem system structure in 15 5 m by 5 m plots I transplanted Ircinia felix into the middle of five plots. I also placed fake sponges in the middle of five plots so that I can tease apart the affect of the sponge from the impact of simply adding structure to the environment. I have hypothesized that seagrass growth will increase, macroalgae will increase in abundance, and the invertebrate community will shift in response to the changes in the primary producers. I’m looking forward to seeing and sharing the results as this project unfolds!