Abstract:
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.
