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Abstract: We present the first measurements of solutes, invertebrates, and microbial activity in the semi-perched hyporheic, parafluvial, and riparian flowpaths of an ephemeral river channel. Dissolved organic carbon (DOC), nitrogen [...]

Two introduced invasive red macroalgae, Eucheuma denticulatum (Burman) Collins & Hervey and Gracilaria salicornia C. Ag., progressively declined in abundance in Ka-ne'ohe Bay, Hawai'i, following ~30 years of increase since their introduction in the mid-1970s. The declines were not the result of biological control or mechanical control efforts, but instead were the result of grazing by native herbivores, probably fishes, as demonstrated by field experiments conducted in 2014 with algal thalli exposed to, and protected from grazing. Two long-term time-series of the abundance of herbivorous fishes at Ka-ne'ohe Bay patch and fringing reef sites indicated that the abundance of herbivorous fishes increased from 2007 through 2014 over the period when these macroalgae declined in cover. Analyses of data of algal cover from surveys of Ka-ne'ohe Bay reefs carried out between 1996 and 2018 indicate that the declines in E. denticulatum and G. salicornia began in approximately 2008, several years before control programs commenced on some of the reefs in the bay. In addition to intense herbivory, high summer water temperatures in 2014 and 2015 may have contributed to the decline in E. denticulatum. Naturally occurring declines in the abundance of alien invasive marine algal species have rarely been reported in the literature. These reductions in macroalgal abundance are the second instance of decline in invasive macroalgae in Ka-ne'ohe Bay since 2006, and have reduced competition between macroalgae and corals on reef flats and reef slopes across the bay. [ABSTRACT FROM AUTHOR]

To meet the challenges of preventing and reversing adverse effects of land use on ecosystems, management actions need to be founded on strong evidence. We used the pressure-state-impact (PSI) framework to assess evidence of land-use effects on New Zealand freshwater ecosystems. The evidence consisted of published quantitative and categorical associations linking land-use pressures to state changes and ecological impacts in rivers, lakes and aquifers. There was substantial evidence of land-use effects, particularly where land use/land cover (LULC) classes were used as pressure variables. Proportions of catchment area in urban and pastoral LULC were consistently, positively correlated with contaminant levels in water bodies and negatively correlated with ecological-health indicators. Other consistent PSI associations included positive correlations between cattle stocking rates and river contaminant levels, increased fine sediment and decreased ecological-health scores in rivers following forest harvest, and increased river contaminant levels at sites with stock access. Despite these consistent associations, the evidence base has four general shortcomings that should be addressed: (1) inadequate integration of data and models that link land use and contaminant loss to state changes and impacts in freshwater ecosystems; (2) weak inferences based on LULC; (3) reliance on categorical PSI associations; (4) gaps in reported PSI associations. [ABSTRACT FROM AUTHOR]

Scientists and environmental managers use stressor-response (S-R) relationships to characterise and predict ecological and socioeconomic responses to anthropogenic stressors in lakes and other aquatic ecosystems. S-R relationships with thresholds (stressor levels beyond which small increases cause large responses) are reported with increasing frequency. Threshold responses pose risks of unanticipated degradation, but also provide a stimulus for shifting environmental management strategies from retrospective (focused on post-hoc responses to degradation) to prospective (focused on preventing degradation). We set out a framework for interpreting S-R relationships in terms of functional forms, trajectories, thresholds and slopes. These characteristics convey information about resistance to degradation and recovery, risks of threshold exceedance, and alternate stable states. We then set out steps for implementing threshold-based management strategies, which are based on forecasting S-R relationships and carrying out preventative actions within an adaptive framework. Prospective, threshold-based management is a challenging yet promising alternative to the prevailing retrospective strategies. [ABSTRACT FROM AUTHOR]

Spatial regression models were used to predict yields (kg ha−1 yr−1) of nitrogen (N) and phosphorus (P) discharged from catchments throughout New Zealand under natural and current conditions. The models were derived using loads (kg yr−1) of TN, NO3-N, TP and DRP calculated for 592 river water quality monitoring sites. Anthropogenic increases in yields above natural levels were associated with the proportions of catchments occupied by the intensive agricultural land cover and were unevenly distributed across regions. Anthropogenic increases in national loads of TN, NO3-N, TP and DRP exported to the ocean were 74%, 159%, 48% and 18%, respectively. Increases in loads exported to the ocean varied considerably at smaller scales, with catchments having significant load increases between 4- and 26-fold for N and 6- to 9-fold for P. Predictions of yields and loads reported here have utility in the development of strategies to manage nutrients. [ABSTRACT FROM AUTHOR]

The stalked diatom Didymosphenia geminata forms proliferations of stalk-dominated mats in rivers. Over its range in New Zealand, mats rarely occur where average dissolved reactive phosphorus (DRP) concentrations exceed ~2 mg m−3, but do occur in rivers with dissolved inorganic nitrogen ( DIN) concentrations up to ~250 mg m−3. Previous studies indicated that low DRP concentrations limit D. geminata cell division and stimulate stalk production and mat development, even at very low ambient DIN concentrations (~10 mg m−3)., We ran two nutrient-enrichment experiments in instream channels in an oligotrophic river with a persistent D. geminata proliferation to (i) test the hypothesis that elevated DRP concentrations reduce or prevent mat development by reducing stalk production; and (ii) investigate the effect of DIN enrichment on D. geminata growth and mat development., In Experiment 1, treatments comprised low-level enrichment with DRP (to 1.6 mg m−3, from superphosphate fertiliser), DIN (to 13.5 mg m−3, from ammonium sulphate), and both DRP and DIN. Compared to unenriched controls, DIN and DRP enrichment enhanced D. geminata cell densities, biomass, cell division (frequency of dividing cells), stalk biovolume and percentage cover of mats, indicating both N- and P limitation for all growth metrics. In the DRP-enriched treatments, biomass accrual may have been stimulated rather than reduced as predicted because DRP concentrations did not exceed the putative 2 mg m−3 threshold., In Experiment 2, we expanded the range of DRP enrichment using additions of orthophosphate as well as superphosphate, and added DIN to all treatments. Higher DRP concentrations (up to 4 mg m−3) initially enhanced D. geminata growth. By the end of the experiment, D. geminata-dominated biomass in the orthophosphate treatments was lower than in the superphosphate or DIN-only treatments., Outflows from the experimental channels caused slight nutrient enrichment in the river downstream. During Experiment 1, this DRP and DIN enrichment had no detectable effect on D. geminata standing crop on the riverbed. During Experiment 2, D. geminata standing crop downstream of the experiment declined relative to the unenriched area upstream., The specific mechanism linking P enrichment to D. geminata biomass decline is unclear. In the experimental channels there was evidence for a predominance of cell division over stalk production in orthophosphate treatments, as hypothesised. In the river, we observed higher rates of sloughing in the river downstream of the experiment than upstream. There was no evidence that competition from other algal taxa reduced D. geminata standing crop., The results supported the hypothesis that DRP enrichment at concentrations over ~2 mg m−3 leads to reduced D. geminata biomass and % cover in this river. However, biomass reduction occurred only when DRP was added as orthophosphate, not as superphosphate., N limitation of D. geminata growth in river water with ambient mean DIN concentrations below ~6 mg m−3 suggests that D. geminata proliferations may be stimulated by increases in DIN concentration in rivers where both DIN and DRP are naturally low. The threshold for N limitation was ~10 mg m−3. [ABSTRACT FROM AUTHOR]