Climatic, oceanic, freshwater, and local environmental drivers of New Zealand estuarine macroinvertebrates.

Understanding the responses of organisms to different environmental drivers is critical for improving ecosystem management and conservation. Estuarine ecosystems are under pressure from multiple anthropogenic stressors (e.g. increasing sediment and nutrient loads, pollution, climate change) that are affecting the functions and services these ecosystems provide. Here, we used long-term estuarine benthic invertebrate monitoring data (∼30 year time-series) to evaluate the responses of macrobenthic invertebrate communities and indicator species to climatic, oceanic, freshwater, and local environmental drivers in New Zealand estuaries. We aimed to improve our ability to predict ecosystem change and understand the effects of multiple environment drivers on benthic communities. Our analyses showed that the abundance and richness of macrobenthic fauna and four indicator taxa (bivalves known to have differing tolerances to sediment mud content: Austrovenus stutchburyi, Macomona liliana, Theora lubrica, and Arthritica bifurca) responded to unique combinations of multiple environmental drivers across sites and times. Macrobenthic responses were highly mixed (i.e., positive and negative) and site-dependent. We also show that responses of macrobenthic fauna were lagged and most strongly related to climatic and oceanic drivers. The way the macrobenthos responded has implications for predicting and understanding the ecological consequences of a rapidly changing environment and how we conserve and manage coastal ecosystems.

Cumulative stressors reduce the self-regulating capacity of coastal ecosystems.

Marine ecosystems are prone to tipping points, particularly in coastal zones where dramatic changes are associated with interactions between cumulative stressors (e.g., shellfish harvesting, eutrophication and sediment inputs) and ecosystem functions. A common feature of many degraded estuaries is elevated turbidity that reduces incident light to the seafloor, resulting from multiple factors including changes in sediment loading, sea-level rise and increased water column algal biomass. To determine whether cumulative effects of elevated turbidity may result in marked changes in the interactions between ecosystem components driving nutrient processing, we conducted a large-scale experiment manipulating sediment nitrogen concentrations in 15 estuaries across a national-scale gradient in incident light at the seafloor. We identified a threshold in incident light that was related to distinct changes in the ecosystem interaction networks (EIN) that drive nutrient processing. Above this threshold, network connectivity was high with clear mechanistic links to denitrification and the role of large shellfish in nitrogen processing. The EIN analyses revealed interacting stressors resulting in a decoupling of ecosystem processes in turbid estuaries with a lower capacity to denitrify and process nitrogen. This suggests that, as turbidity increases with sediment load, coastal areas can be more vulnerable to eutrophication. The identified interactions between light, nutrient processing and the abundance of large shellfish emphasizes the importance of actions that seek to manage multiple stressors and conserve or enhance shellfish abundance, rather than actions focusing on limiting a single stressor.

Mapping the estuarine ecosystem service of pollutant removal using empirically validated boosted regression tree models.

Humans rely on the natural environment and benefit from the goods and services provided by natural ecosystems. Quantification and mapping of ecosystem services (ES) is required to better protect valued ES benefits under pressure from anthropogenic activities. The removal of excess nitrogen, a recognized catchment-derived pollutant, by biota in estuarine soft sediments is an important ES that potentially ameliorates the development of eutrophication symptoms. Here, we quantified estuarine benthic sediment characteristics and denitrification enzyme activity (DEA), a proxy of inorganic N removal, at 109 sites in four estuaries to develop a general ("global") model for predicting DEA. Our initial global model for linking DEA and environmental characteristics had good explanatory power, with sediment mud content having the strongest influence on DEA (60%), followed by sediment organic matter content (≈35%) and sediment chlorophyll a content (≈5%). Predicted and empirically evaluated DEA values in a fifth estuary (Whitford, n = 90 validation sites) were positively correlated (r = 0.77), and the fit and certainty of the model (based on two types of uncertainty measures) increased further after the validation sites were incorporated into it. The model tended to underpredict DEA at the upper end of its range (at the muddier, more organically enriched sites), and the relative roles of the three environmental predictors differed in Whitford relative to the four previously sampled estuaries (reducing the explained deviance relative to the initial global model). Our detailed quantification of DEA and methodological description for producing empirically validated maps, complete with uncertainty information, represents an important first step in the construction of nutrient pollution removal ES maps for use in coastal marine spatial management. This technique can likely be adapted to map other ecosystem functions and ES proxies worldwide.

Biodiversity breakpoints along stress gradients in estuaries and associated shifts in ecosystem interactions.

Denitrification in coastal sediments can provide resilience to eutrophication in estuarine ecosystems, but this key ecosystem function is impacted directly and indirectly by increasing stressors. The erosion and loading of fine sediments from land, resulting in sedimentation and elevated sediment muddiness, presents a significant threat to coastal ecosystems worldwide. Impacts on biodiversity with increasing sediment mud content are relatively well understood, but corresponding impacts on denitrification are uncharacterised. Soft sediment ecosystems have a network of interrelated biotic and abiotic ecosystem components that contribute to microbial nitrogen cycling, but these components (especially biodiversity measures) and their relationships with ecosystem functions are sensitive to stress. With a large dataset spanning broad environmental gradients this study uses interaction network analysis to present a mechanistic view of the ecological interactions that contribute to microbial nitrogen cycling, showing significant changes above and below a stressor (mud) threshold. Our models demonstrate that positive biodiversity effects become more critical with a higher level of sedimentation stress, and show that effective ecosystem management for resilience requires different action under different scenarios.

In situ soft sediment nutrient enrichment: A unified approach to eutrophication field experiments.

Adding fertiliser to sediments is an established way of studying the effects of eutrophication but a lack of consistent methodology, reporting on enrichment levels, or guidance on application rates precludes rigorous synthesis and meta-analysis. We developed a simple enrichment technique then applied it to 28 sites across an intertidal sandflat. Fertiliser application rates of 150 and 600gNm(-2) resulted in pore water ammonium concentrations respectively 1-110 and 4-580×ambient, with greater elevations observed in deeper (5-7cm) than surface (0-2cm) sediments. These enrichment levels were similar to eutrophic estuaries and were maintained for at least seven weeks. The high between-site variability could be partially explained by the sedimentary environment and macrofaunal community (42%), but only at the high application rate. We suggest future enrichment studies should be conducted in situ across large environmental gradients to incorporate real world complexity and increase generality of conclusions.