Interactions between Sponges and the Water Column: Nutrient Utilisation and Feeding by New Zealand Subtidal Sponges
Sponges are an important component of New Zealand subtidal communities and play many key functional roles in marine ecosystems, including competition for space, facilitating primary production, nutrient cycling, bioerosion, and interactions with the water column. Sponges are involved in the bidirectional movement of detritus, nutrients, micro-organisms and planktonic particles both to and from the benthos to the pelagic ecosystem, thereby affecting pelagic processes. As suspension-feeders, sponges are capable of filtering large volumes of water, and they depend on food that is suspended in the water column, meaning that their interaction with the water column is likely to be very important. The main goal of my research was to investigate the interactions between sponges and the water column and how this varies in relation to sponge characteristics, nutrient fluxes, seasonality and food supply. I studied the diet composition of 10 sponge species that are abundant and widely distributed along the south coast of Wellington, New Zealand. I found that the diet of the sponge species analysed comprised three types of picoplanktonic organisms: heterotrophic bacteria, Prochlorococcus, and Synechococcus. These micro-organisms (picoplankton) that sponges feed on are vital for benthic food webs because they are involved in the transformation and cycling process of dissolved inorganic nutrients before they become available to other marine organisms. The results from this thesis demonstrated that different sponge species have different retention efficiencies for different types of picoplankton and I propose that this suggests intra-phyletic food particle niche partitioning among sponges. While these findings support the partitioning of food resources between different co-existing sponge species, they also suggest that partitioning may not be essential for co-existence, as some species had similar retention efficiencies implying an overlap in resource use. By measuring rates of carbon assimilation in the form of planktonic food particles, combined with data on a number of characteristics of the sponge species analysed, I found that sponge assemblages play a key role in the transfer of energy from the water column to the benthos. The results from this thesis indicate that there is a wide range of food concentrations in the rocky reefs where the study species are living, over which retention rate, nutrient utilisation and carbon consumption varied temporally. This emphasises the importance of understanding temporal variation in productivity, and suggests that such variations are likely to have important implications for suspension-feeders. By integrating the feeding results with estimations of oxygen consumption rates, and the amount of carbon obtained from the different micro-organisms found in the water column, preliminary carbon budgets were created. These budgets were used to quantify the capacity of carbon obtained via heterotrophic suspension-feeding to support sponge metabolism, as well as infer the potential for this carbon to support other processes such as sponge growth and reproduction. Overall, this project was the first to consider the functional roles of sponges in New Zealand marine ecosystems and provided useful information on their ecological and biological importance. The large amounts of carbon that sponges transfer from the water column to the benthos, in conjunction with the other findings of my thesis, increase our understanding of the ecology of temperate sponges.