Sinks of agriculturally derived nitrogen in estuarine and coastal lagoon ecosystems

Abstract

Increased nutrient runoff from mismanaged agricultural farming results in excess inputs of nutrients to coastal ecosystems, which can increase primary productivity causing eutrophication. Estuarine sediments provide the ecosystem service of nutrient cycling; one of the most important of which is denitrification. Denitrification is a key process of the nitrogen cycle which occurs at the interface of oxic/anoxic sediments, where bioavailable nitrate is converted to nitrogen gas and released into the atmosphere. This process filters excess nutrients, reducing bioavailable nitrogen before export to the ocean. However, the process of denitrification is still a limited area of research in New Zealand.

This thesis aims to elucidate some of the drivers of spatial and seasonal variability of denitrification predominantly focussing on a unique type of estuary, intermittently closed and open lake lagoons (ICOLLs). Firstly, the seasonality of nutrient inputs is investigated, to see what role that may play in shaping the denitrification potential of the microbial community across two contrasting ICOLLs using denitrification enzyme activity (DEA) (Chapter 2). Secondly, spatial and seasonal variability of denitrification is assessed using novel flexible chambers in situ, under pulses of labelled nitrate (K15NO3-) which was identified to occur in the second chapter (Chapter 3). As well as seasonality in temperature, seasonality in the abundance of dominant macroinvertebrates also changes, and the impacts of bioturbation on denitrification was assessed in situ over a year-long seasonal study in a small ICOLL (Chapter 4). As many estuarine ecosystems are beginning to show eutrophication symptoms (such as a decrease in stable rooted macrophytes and an increase in bloom forming macro- and micro- algae), the effects of changing organic detritus on net nitrogen fluxes was examined in a low nutrient estuary, which periodically experiences blooms of algae (Chapter 5). These results are synthesised and discussed in Chapter 6.

The two studied ICOLLs (Ellesmere and Tomahawk) periodically received large inputs of nitrogen (>0.3 mg L-1 NO3-), which mostly occurred during spring and winter. Denitrification potential in the ICOLLs was strongly correlated with the sediment grain size, rather than near these large nutrient inflows as hypothesised. Denitrification potential was greater in sediments with higher sand content. A hierarchy of factors limited denitrification in situ. Primarily, temperature inhibited denitrification at low temperatures, occurring between 8.6 - 12°C. Following this the availability of organic matter, shallow water depth and macroinvertebrate abundance supported increased denitrification rates. Macroinvertebrate communities supported enhanced denitrification and sediment oxygenation. Potamopyrgus antipodarum provided bulldozing of the upper sedimentary layers, and burrowing chironomid larvae increased the extent of the oxic/anoxic sediment interface for denitrification in deeper sediment layers.

The lability of various forms of organic carbon played an indirect role in regulating the switch between net N2 removal through denitrification, or nitrogen retention through nitrogen fixation. Cockles reduced nitrogen and sulfur enzyme activity, through changing oxygen conditions and increasing substrate supplies to denitrifying microorganisms. Minimising anthropogenic stressors such as sedimentation, managed ICOLL opening regimes, and nitrogen loading during cooler months will support positive ecosystem processes and the preservation of our unique estuarine ecosystems for future generations.