Joanne E. Clapcott

Forest clearance increases metabolism and organic matter processes in small headwater streams

Abstract Small headwater streams are abundant components of the riverine landscape where critical biochemical processes occur that provide clean water, energy, and nutrients to downstream reaches. Disturbance to these systems as a result of human land use has the potential to affect downstream health. Rates of metabolism and organic matter processing were measured in 22 small forested headwater streams in 2 regions of Tasmania, Australia, to evaluate the effects of forestry disturbance. Twelve of these streams had been subjected to recent clearfell-burn-and-sow (CBS) harvest. Benthic metabolism was measured in small in situ chambers (production ranged from <0.001 to 21.845 mg C m−2 h−1 and respiration from <0.001 to 4.976 mg C m−2 h−1), whole-system metabolism was estimated based on relative habitat abundance (gross primary production ranged from <0.001 to 0.297 g C m−2 d−1 and daily respiration from 0.003 to 0.072 g C m−2 d−1). Algal growth potential was measured on nutrient diffusing pots (chlorophyll a ranged from <1.0 to 40.1 mg/m2), and cellulose decomposition potential was assessed with a cotton-strip assay (cotton tensile strength loss ranged from 17.8% to 38.3% in 28 d). Sometimes an increase in the variability of response is a consequence of disturbance, but in our study, the difference between forested streams and clearcut streams was a significant increase in the mean values of all functional variables. The degree of response depended on the underlying geology (broad-scale spatial variability) of the streams. Current management practices for small headwater streams in Tasmania do not protect instream processes from forestry disturbance in the short-term (i.e., 2–5 y), and we suggest that an investigation of long-term response is warranted.

Identifying congruence in stream assemblage thresholds in response to nutrient and sediment gradients for limit setting

The setting of numeric instream objectives (effects-based criteria) and catchment limits for major agricultural stressors, such as nutrients and fine sediment, is a promising policy instrument to prevent or reduce degradation of stream ecosystem health. We explored the suitability of assemblage thresholds, defined as a point at which a small increase in a stressor will result in a disproportionally large change in assemblage structure relative to other points across the stressor gradient, to inform instream nutrient and sediment objectives. Identification and comparison of thresholds for macroinvertebrate, periphyton, and bacterial assemblages aimed at making the setting of objectives more robust and may further provide a better understanding of the underlying mechanisms of nutrient and fine sediment effects. Gradient forest, a novel approach to assemblage threshold identification based on regression-tree-based random forest models for individual taxa, allowed inclusion of multiple predictors to strengthen the evidence of cause and effect between stressors and multispecies responses. The most prominent macroinvertebrate and periphyton assemblage threshold across the nitrogen (N) gradient was located at very low levels and mainly attributed to declines of multiple taxa. This provided strong evidence for stream assemblages being significantly affected when N concentrations exceed reference conditions and for effects cascading through the ecosystem. The most prominent macroinvertebrate assemblage threshold across a gradient of suspended fine sediment was also located at very low levels and attributed to declines of multiple taxa. However, this threshold did not correspond with periphyton assemblage thresholds, suggesting that the sensitivity of macroinvertebrate assemblages is unrelated to sediment effects on periphyton assemblages. Overall, the spectrum of N concentrations and fine sediment levels within which these stream assemblages changed most dramatically were relatively narrow given the wide gradients tested. We conclude that assemblage thresholds can inform the setting of generic instream nutrient and sediment objectives for stream ecosystem health. For example, the most stringent objective for instream N concentration should be set at values similar to reference concentrations for full protection of sensitive taxa or overall stream biodiversity. To avoid severe degradation of stream biodiversity, the least stringent N objective should stay well below the point where significant turnover subsided.

Relationships between ecosystem metabolism, benthic macroinvertebrate densities, and environmental variables in a sub-arctic Alaskan river

Relationships between environmental variables, ecosystem metabolism, and benthos are not well understood in sub-arctic ecosystems. The goal of this study was to investigate environmental drivers of river ecosystem metabolism and macroinvertebrate density in a sub-arctic river. We estimated primary production and respiration rates, sampled benthic macroinvertebrates, and monitored light intensity, discharge rate, and nutrient concentrations in the Chena River, interior Alaska, over two summers. We employed Random Forests models to identify predictor variables for metabolism rates and benthic macroinvertebrate density and biomass, and calculated Spearman correlations between in-stream nutrient levels and metabolism rates. Models indicated that discharge and length of time between high water events were the most important factors measured for predicting metabolism rates. Discharge was the most important variable for predicting benthic macroinvertebrate density and biomass. Primary production rate peaked at intermediate discharge, respiration rate was lowest at the greatest time since last high water event, and benthic macroinvertebrate density was lowest at high discharge rates. The ratio of dissolved inorganic nitrogen to soluble reactive phosphorus ranged from 27:1 to 172:1. We found that discharge plays a key role in regulating stream ecosystem metabolism, but that low phosphorous levels also likely limit primary production in this sub-arctic stream.

Finding reference: a comparison of modelling approaches for predicting macroinvertebrate community index benchmarks

ABSTRACT Reference benchmarks are needed to assess the contemporary status of rivers and to establish restoration targets. We developed predictive models to estimate site-specific reference values for a macroinvertebrate community index (MCI), which is used to indicate a range of human impacts on wadeable streams. We compared three statistical modelling approaches – general linear, boosted regression tree (BRT) and random forest (RF) – and tested the effect of spatial scale on predictive accuracy by developing national and regional BRT models. Using fitted flexible models (BRT, RF) and resetting predictors to reflect natural state provided the most accurate predictions of reference condition. Variation in reference MCI predictions from national and regional models was within the range observed from methodological and temporal variability. The proportion of native vegetation in upstream catchments was the primary predictor of MCI scores in all models, while secondary predictors varied regionally.

Land use affects temporal variation in stream metabolism

Stream metabolism (gross primary production and ecosystem respiration) is increasingly used to assess waterway health because mean values are responsive to spatial variation in land use, but little is known about how human land use influences the temporal variability of stream metabolism. We investigated daily variation in dissolved O2 (DO) concentrations and calculated mean and within-season variation in gross primary production (GPP) and ecosystem respiration (ER) rates at 13 stream sites across a landuse intensity gradient in the Auckland region, New Zealand, over 9 y. Based on generalized linear mixed models, mean daily GPP (0.1–12.6 g O2 m−2 d−1) and ER (1.8–29.6 g O2 m−2 d−1) and seasonal variation in stream metabolism were significantly related to landuse intensity with higher variability associated with higher values of a landuse stress score. Overall, mean daily rates and day-to-day variation in GPP and ER were greatest in summer and least in winter. We recommend summer monitoring over a minimum 5-d period to assess stream health. Our results show that human land use affects the mean and the temporal variability of DO and stream metabolism. This finding has important consequences for characterizing in-stream processes and the resilience of stream ecosystems. Only long-term temporal monitoring provides the data needed to assess fully how streams function.

Thresholds in ecosystem structural and functional responses to agricultural stressors can inform limit setting in streams

Setting numeric in-stream objectives (limits, criteria) to inform limits on catchment loads for major land-use stressors is a promising policy instrument to prevent ecosystem degradation. Management objectives can be informed by thresholds identified from stressor–response shapes of ecological indicators based on field survey data. Use of multiple structural and functional indicators and different organism groups provides multiple lines of evidence to make objectives more robust. We measured a suite of ecological indicators during a regional field survey in New Zealand. We built flexible boosted regression tree (BRT) models with a predictor set consisting of nutrient, sediment, and environmental variables and investigated the fitted functions for different types of thresholds across each stressor gradient. Congruence of impact initiation (II) thresholds for N among macroinvertebrate metrics and 2 periphyton indicators provided multiple lines of evidence for ecosystem change with small increases in N concentrations above background levels. Impact cessation (IC) on macroinvertebrate metrics at total N = ~0.5 mg/L (below N concentrations that saturate important ecosystem processes) highlighted sensitivity of macroinvertebrate communities to eutrophication. We found few stressor–response relationships for sediment. We suggest use of sediment-specific macroinvertebrate metrics and a reliable measure of deposited fine sediment in the future. Few indicators responded to phosphorus (P) concentration. Limited information for setting P objectives highlights the need to develop alternative indicators of P loading. Statistical analysis based on single-stressor inferential threshold models suggested that these models carry high risk of identifying spurious thresholds and are less suitable for setting management objectives. II and IC thresholds of multiple ecological indicators can be used to set robust objectives aimed at different levels of protection of ecosystem health.

Forestry affects the abundance of Phormidium-dominated biofilms and the functioning of a New Zealand river ecosystem

Toxic cyanobacterial proliferations in water bodies can cause serious environmental and public health issues, as well as having economic effects. Increased inputs of nutrients and fine sediment caused by forestry have been hypothesised as possible causes of increased Phormidium-dominated proliferations in New Zealand rivers. Little is known about the effect of these proliferations on river ecosystem functioning. In the present study, we evaluated five sites along the Maitai River (New Zealand) differing in pine plantation cover of their catchments. We hypothesised that Phormidium biofilms would trap more sediments and recycle more phosphorus than diatoms, that Phormidium proliferations would increase with forestry cover in the catchment and that the varying abundance of Phormidium would affect river ecosystem functioning. Phormidium did not trap more sediment or recycle more phosphorus (measured as alkaline phosphate activity) than diatom biofilms. However, the cover of Phormidium did increase with the proportion of forestry in the catchment. Organic matter decomposition rates (measured as loss of tensile strength of standard cotton strips) varied very little among sites, whereas river ecosystem metabolism increased with the abundance of Phormidium, especially in the lower part of the river. The results of the present study suggest that pine forestry does promote Phormidium biofilm abundance and affect ecosystem functioning in the Maitai River.