Influence of catchment land-use intensity on macroinvertebrates and ecosystem function along a freshwater-marine continuum

Worldwide catchment land-use intensity, especially for agriculture, is increasing in order to meet the food demands of the growing human population. Besides their numerous benefits for humans, expansion of agricultural and urban areas and intensification of catchment land use also threaten human well-being, ecosystems and biodiversity due to land degradation and deterioration of global water quality. Conversion from native vegetated land to agricultural land and urban areas often result in increased inputs of nutrients, fine sediment and pollutants such as pesticides and heavy metals into running waters, lakes and eventually estuaries, influencing their ecological health. In response to the worsening environmental conditions, freshwater and estuarine communities have undergone compositional changes, either declining in overall diversity or increasing in abundance of r-selected, tolerant species, and thereby modifying trophic interactions. The overall aim of my study was to investigate whether the influence of catchment land-use intensity on macroinvertebrate community composition and ecosystem function decreases or accumulates along a freshwater-marine continuum. A survey of 21 lowland rivers and their estuaries in the South Island of New Zealand revealed marked shifts in invertebrate community and functional traits with increasing catchment land-use intensity and associated nutrient and sediment inputs. At the lowland river sites, the macroinvertebrate community changed from pollution-sensitive taxa such as Ephemeroptera, Plecoptera and Trichoptera to pollution-tolerant taxa such as Amphipoda, Diptera, Mollusca and Oligochaeta. Further downstream, in the estuaries of these lowland rivers, a similar shift towards pollutant-tolerant species such as Polychaeta, Oligochaeta, Bivalvia and Gastropoda in developed catchments was observed, while taxon richness in general decreased with increasing catchment development. Along the freshwater-marine continuum, the prevalence of trait categories associated with high resilience and resistance increased in parallel with catchment development, while traits sensitive to catchment land-use intensification (e.g. increasing fine sediment levels) declined. In an experimental study, I investigated the influence of catchment development on two key ecosystem function parameters, algal biomass accrual and organic matter decomposition along the freshwater-marine continuum in a subset of 10 rivers and estuaries out of the 21 studied systems in my survey. Algal biomass accrual and cellulose decomposition rates generally increased in parallel with nutrient concentrations, but were adversely affected by the direct influence of catchment development. Additionally, I tested the performance of a standard cotton material recently proposed in the literature as a new bioassay for detecting changes in decomposition rates along a gradient of catchment land use intensity. For detecting direct influences of land-use intensity expressed as % pristine catchment area, both tested materials, canvas and cotton, performed similarly, being negatively related to catchment development. The results of both field surveys and experiments imply that (1) the effects of catchment development did not decrease gradually towards the ocean and (2) multiple stressors such as nutrient enrichment and sediment augmentation, but also physical disturbance, toxicants and other factors outside the scope of the study, influence the ecological health of lowland rivers and estuaries in Southern New Zealand. Therefore, further research is needed to identify and disentangle the individual and combined effects of these stressors to minimize negative effects of intensive catchment land uses on both rivers and estuaries