Several of the student projects have been related to NIWA’s large Sustainability of Cultured Fisheries programme. This has the very practical goal of determining the extent to which aquaculture can be expanded in the Marlborough Sounds without deleterious effects, and also, will help with understanding how nutrients flow through a nearshore marine system.
The enclosed nature of the Marlborough Sounds is an ideal environment to analyse such ecosystem processes and allows CEAME to train students in oceanographic techniques without the expensive vessels and gear required for work in the open ocean.
Doing experiments in natural embayments is challenging because most of the variables of importance operate at large scales through complex ecosystem pathways. One way to sort through these processes experimentally is to use “mesocosms”. These are essentially very large plastic enclosures that are suspended from floating collars from the sea surface and extend sometimes to the seabed. The seawater within the mesocosms can then be subjected to different experimental treatments such as added nutrients, additional animal activity and varied light regimes.
Examples of mesocosm experiments include:
Nitrogen is a natural fertiliser of most New Zealand coastal systems, and is variable in supply throughout the year but is essential to phytoplankton (microscopic algae) growth.
Mussels feed almost entirely on these small plants so their growth and the overall production of the mussel industry rely on these processes. Adding to the complexity are zooplankton (microscopic animals) that graze on phytoplankton, potentially competing with mussels, but are themselves eaten by larger predators.
One CEAME project used replicate mesocosms (link above to mesocosm experiments) holding 850 litres of seawater each to test the effects of different nitrogen concentrations on phytoplankton production with and without mussels feeding. In winter, when there were adequate nutrients in the sea, additional nitrogen had no effect on phytoplankton production but in summer, when nitrogen in the sea was in short supply, additional nitrogen, even in small quantities, had a significant effect on phytoplankton and therefore on mussel food. Combined with physical measurements of current flow and direction, these sorts of experimental results lead to predictions of overall food production and food depletion by mussels in coastal embayments.
Mesocosm experiments were used in another CEAME project to test the extent to which zooplankton compete with mussels for food in embayments. Zooplankton and mussels were added in differing ratios to mesocosms and the effect on phytoplankton was studied. The results were surprising, showing processes in the plankton were more complicated than first thought. For instance, even jellyfish will play a significant role by removing larger zooplankton, which in turn affect smaller zooplankton which feed on phytoplankton. These so-called trophic processes, therefore, can affect mussel food production.