Abstract
Developing robust means of identifying anthropogenic stressor levels that degrade aquatic ecosystems represents a global management and conservation challenge. Among the known threats for coastal biodiversity and ecosystem processes, diffuse inputs of nitrogen (N) from intensive agriculture are important stressors of estuarine ecosystems. Due to on-going land use intensification, N pollution of estuaries is expected to increase, and there is an urgent need to improve understanding and assessment of the consequences of N enrichment for estuarine ecosystems. In this thesis, I investigated the effects of increased catchment-derived N loading on key benthic biota (macroinvertebrates, macroalgae and seagrass) to identify their efficacy as ecological indicators. The potential for another stressor (sedimentation) to confound relations between N loading and biota was also assessed. To achieve this, data were collected from multiple shallow, intertidal-dominated estuaries (SIDEs) throughout New Zealand subject to different catchment-derived N loading regimes.
Macroinvertebrate communities were surveyed with a suite of sedimentary abiotic parameters at representative sites in eight geographically distinct SIDEs. A subset of the abiotic parameters were associated with modelled catchment N loading rates and these were significant drivers of benthic macroinvertebrate assemblages. Modelling of taxon-specific (abundance maxima) responses along a gradient of sediment N enrichment enabled categorisation into ecological groups, which significantly strengthened the predictive power of an existing benthic index, the ASTI (ASTI-Tecnalia Marine Research Division, Spain) Marine Benthic Index (AMBI) to differentiate stress related to N enrichment and sediment mud content. An important result to inform monitoring strategy is that integrated 0 – 15 cm down-core sampling depth was a better indicator of eutrophication status than 0 – 2 cm. Further, long-term monitoring sites should be established in upper and lower zones of SIDEs to reflect apparent differences in susceptibility/response to N loading.
Eutrophication of shallow estuaries often manifests as dense mats of opportunistic macroalgae, which alter sediment chemistry and benthic infaunal communities, and displace other important primary producers including seagrass beds. To quantify stressor levels responsible for seagrass—macroalgal shifts, I investigated relationships between catchment-derived N and suspended sediment (SS) loads and the spatial extent of seagrass and nuisance-level macroalgae, using broad-scale GIS habitat data for twenty-five SIDEs characterised by different N loading regimes. Catchment N and SS loads were not correlated in this study, allowing me to discern their individual and combined effects. The spatial extent of both macrophyte types were strongly governed by increasing N loading. Seagrass extent declined significantly as N loading rates increased towards critical values, but once surpassed, became abruptly spatially restricted or absent. Conversely, nuisance macroalgae were predominantly absent unless loading rates were relatively high, expanding in extent once loads increased above critical levels. SS loading was not a significant predictor of the extent of either macrophyte type. In addition, dramatic hysteresis in seagrass loss and nuisance macroalgal production as a function of N loading was revealed, based on changes over 16 years in four SIDEs subject to different N loads.
Once established in estuaries, the persistence of macroalgae may be fuelled by elevated nutrient availability. Sediment porewater nutrients often occur at concentrations that are orders of magnitude higher than nutrients in overlying waters in estuaries. The relative contribution of porewater nutrients is expected to be particularly important for macroalgae entrained in eutrophic intertidal mudflat sediments, where access to water column nutrients is tidally constrained. Hence, thalli of filamentous Gracilaria chilensis (a nuisance macroalgae in the study SIDEs) were simultaneously exposed to sediment and bottom water nutrient sources, labelled using 15N tracers. Dissolved inorganic N (DIN) uptake from porewater and bottom water accounted for similar proportions of the N necessary to support the growth of G. chilensis, despite the two-fold lower DIN concentration of the overlying water and its periodic availability. Ammonia was preferentially assimilated over nitrate. This ability to utilise multiple sources and species of N relatively rapidly may partly explain the competitive advantage and persistence of opportunistic macroalgae over historically abundant seagrass beds in these environments. Furthermore, these results underscore the significance of both internal N loading and external inputs as important in sustaining nuisance macroalgal blooms in this class of shallow estuary.
This thesis demonstrates the efficacy of certain biological and physicochemical parameters to quantitatively indicate stress related to N loading in SIDEs. It confirms N loading as a dominant catchment-scale stressor of shallow intertidal estuaries in New Zealand. My findings suggest that areal N loading rates entering estuaries from surrounding catchments should not exceed approximately 50 – 100 mg N m-2 d-1 if managers wish to maintain healthy macroinvertebrate assemblages as well as limit the occurrence of nuisance macroalgae and not exceed approximately 20 mg N m-2 d-1 to avoid marked losses of seagrass habitat.