Abstract:
Estuaries are some of our most economically and ecologically valuable natural systems; however, human propensity to settle along coasts makes estuaries vulnerable to anthropogenic stress, and nutrient pollution has caused wide-spread eutrophication in many of the world’s estuaries. While many eutrophication mitigation strategies are being investigated and deployed in many of these chronically impacted systems, New Zealand’s estuaries have remained largely oligotrophic. However, nitrogen inputs have increased more in New Zealand than anywhere else in the world, and an estimated 27 % of New Zealand estuaries are now considered susceptible to eutrophication. In addition to this increasing threat, many New Zealand estuaries are also under the threat of increasing sediment deposition as a result of increasing population and land-use change.
In this dissertation research, I sought to understand how this increasing terrestrial sediment deposition is impacting the ecology and biogeochemistry of these low-nutrient ecosystems, and to investigate how they intersect to influence the delivery of the key ecosystem service of denitrification. Due to its ability to mitigate nitrogen pollution, denitrification will be of particular importance to New Zealand ecosystems as nitrogen pollution continues to rise.
Using a combination of literature analysis, in situ field studies, controlled laboratory experiments, and multi-variate modelling approaches, I have demonstrated the importance of these understudied, low-nutrient systems, showing that they function differently than their more eutrophic counterparts. I found that the macrofaunal community is inexorably linked to the biogeochemical processing in these systems, exhibiting direct control on rates of net denitrification. Embracing more complex and non-linear stress responses, I have shown that even small changes in anthropogenic impact can induce regime shifts within these systems. Overall, results from this work show that terrestrial sediment deposition is directly affecting the macrofaunal community and decreasing denitrification rates, thus impairing the ability of these important ecosystems to mitigate ever increasing nitrogen pollution. It is my hope that interdisciplinary studies like the ones in this dissertation can be used to inform more wholistic and effective management of estuarine systems in the future, hopefully keeping eutrophication from becoming an inevitability in New Zealand.