Heatwaves and carbon dioxide enrichment impact invertebrate drift and insect emergence patterns across time in experimental streams.

Climate change and human land use are considered key threats to freshwater invertebrates. Heatwaves can impact the phenology of insects and population dynamics, yet have been largely ignored in experiments compared to mean temperature changes. Another major anthropogenic stressor driving invertebrate community changes is deposited fine sediment; therefore, effects of key climate-change drivers on invertebrate drift and insect emergence rates may differ between sediment-impacted and non-impacted streams. However, this has never been tested in a realistic outdoor experiment. We investigated the individual and combined effects of two 7-day heatwaves, CO2 enrichment, flow velocity variability (periods of fast and slow) and fine sediment on stream drift and emergence responses, sampled four times during a 7-week experiment in 128 flow-through stream mesocosms. We examined invertebrate drift and insect emergence responses to the four stressors, and used these responses to help explain the benthic invertebrate community responses already assessed (sampled at the end of the experiment). Heatwave 1 strongly increased emergence (dominated by Chironomidae), causing an earlier emergence peak, an effect not repeated during heatwave 2, seven days later. During heatwave 1, emerged chironomids were larger in heated channels, but smaller in heated channels afterwards, suggesting a different effect on body size of short-term heatwaves to previous constant warming experiments. CO2 enrichment reduced drifting EPT and total and Chironomidae emergence on three sampling occasions each. After heatwave 1, total drift and total emergence were strongly reduced by heating in ambient-CO2 channels, whereas no reduction occurred in CO2-enriched channels. During heatwave 2, total drift increased in channels without sediment but not in channels with added sediment. Overall, our findings suggest heatwaves can shift the timing of stream insect emergence, regardless of longer-term mean temperatures. They also show that heatwaves, raised CO2, and fine sediment can modulate each others' effects on drift and emergence dynamics.

Biogeographical patterns of species richness in stream diatoms from southwestern South America.

The latitudinal diversity gradient (LDG) hypothesis has been validated for many taxon groups, but so far, stream diatoms have not conformed to this pattern. Research on diatoms that includes data from South America is lacking, and our study aims to address this knowledge gap. Previous studies have successfully explained stream diatom species richness by considering niche dimensionality of physicochemical variables. Moreover, in southwestern South America, the observed biogeographical pattern differs from LDG and has been shown to be determined by historical factors. We used a dataset comprising 373 records of stream diatom communities located between 35° S and 52° S latitude, southwestern South America. The dataset included physicochemical river water variables, climate data, and ice sheet cover from the Last Glacial Maximum. We explored geographical patterns of diatom species richness and evaluated 12 different causal mechanisms, including climate-related theories, physicochemical and climatical exploratory analyses, historical factors, and niche dimensionality. A metacommunity analysis was conducted to evaluate the possible nested structure due to historical factors. We observed an increase in diatom species richness from south to north. Models containing both physicochemical and climatic predictors explained the highest proportion of variation in the data. Silica, which was correlated with latitude, and flow velocity, which did not show any spatial pattern, were the most important predictors. Historical factors and nested structure did not play any role. Contrary to what has been reported in the literature, we found no support for climate-related explanations of species richness. Instead, theories related to niche dimensionality and local factors provided better explanations, consistent with previous related research. We suggest that the increase in diatom richness in the north of our study region is due to a higher nutrient supply in these rivers, rather than a due to larger species pool in the area.

Freshwater invertebrate responses to fine sediment stress: A multi-continent perspective.

Excessive fine sediment (particles <2 mm) deposition in freshwater systems is a pervasive stressor worldwide. However, understanding of ecological response to excess fine sediment in river systems at the global scale is limited. Here, we aim to address whether there is a consistent response to increasing levels of deposited fine sediment by freshwater invertebrates across multiple geographic regions (Australia, Brazil, New Zealand and the UK). Results indicate ecological responses are not globally consistent and are instead dependent on both the region and the facet of invertebrate diversity considered, that is, taxonomic or functional trait structure. Invertebrate communities of Australia were most sensitive to deposited fine sediment, with the greatest rate of change in communities occurring when fine sediment cover was low (below 25% of the reach). Communities in the UK displayed a greater tolerance with most compositional change occurring between 30% and 60% cover. In both New Zealand and Brazil, which included the most heavily sedimented sampled streams, the communities were more tolerant or demonstrated ambiguous responses, likely due to historic environmental filtering of invertebrate communities. We conclude that ecological responses to fine sediment are not generalisable globally and are dependent on landscape filters with regional context and historic land management playing important roles.

Monthly sampling reveals seasonal fine sediment fluctuations and riverine invertebrate community responses.

Managing the impacts of anthropogenically enhanced deposited fine sediment levels in lotic ecosystems requires understanding of how catchment land-use changes have altered the natural sediment regime (erosion, transport, deposition) of rivers. Unfortunately, no existing studies have employed an appropriate sampling frequency over a period encompassing the full range of seasonal flow conditions expected to influence in-stream sediment dynamics. We determined the short-term (monthly) dynamics of deposited fine sediment and invertebrate communities over 12-months in 15 fourth- and fifth-order rivers draining catchments of low, medium and high land-use intensity in Southland, New Zealand to determine when and where fine sediment threatens stream health. We compared the Quorer resuspension method (suspendable inorganic sediment, SIS) and the in-stream visual sediment cover assessment method, and evaluated the effectiveness of four commonly-used invertebrate stream health metrics against their newly developed sediment-specific counterparts. Monthly variability in SIS was substantial across all land-use categories, but became more pronounced as land-use intensity increased. All 15 sites experienced a prolonged period of relatively stable flow which coincided with the largest short-term increase in SIS at 14 of the 15 sites. However, variability in SIS was not mirrored in macroinvertebrate metrics. These findings suggest that controlling inputs of fine sediment to rivers and streams will be most effective when targeted at periods of prolonged stable flow, particularly within high land-use intensity catchments. The resuspension method consistently outperformed visual estimates when considering its relationship with macroinvertebrate metrics, while sediment-specific metrics demonstrated a stronger association with fine sediment than commonly employed metrics e.g. (%EPT). We conclude that restoration/mitigation practices cannot be based solely on short-term, or even long-term, reductions in fine sediment, or on physical measures alone, but should be based on long-term recoveries of sediment-impacted invertebrate communities using concurrent measurements of both biotic and abiotic conditions.

Perceived multiple stressor effects depend on sample size and stressor gradient length.

Multiple stressors are continuously deteriorating surface waters worldwide, posing many challenges for their conservation and restoration. Combined effect types of multiple stressors range from single-stressor dominance to complex interactions. Identifying prevalent combined effect types is critical for environmental management, as it helps to prioritise key stressors for mitigation. However, it remains unclear whether observed single and combined stressor effects reflect true ecological processes unbiased by sample size and length of stressor gradients. Therefore, we examined the role of sample size and stressor gradient lengths in 158 paired-stressor response cases with over 120,000 samples from rivers, lakes, transitional and marine ecosystems around the world. For each case, we split the overall stressor gradient into two partial gradients (lower and upper) and investigated associated changes in single and combined stressor effects. Sample size influenced the identified combined effect types, and stressor interactions were less likely for cases with fewer samples. After splitting gradients, 40 % of cases showed a change in combined effect type, 30 % no change, and 31 % showed a loss in stressor effects. These findings suggest that identified combined effect types may often be statistical artefacts rather than representing ecological processes. In 58 % of cases, we observed changes in stressor effect directions after the gradient split, suggesting unimodal stressor effects. In general, such non-linear responses were more pronounced for organisms at higher trophic levels. We conclude that observed multiple stressor effects are not solely determined by ecological processes, but also strongly depend on sampling design. Observed effects are likely to change when sample size and/or gradient length are modified. Our study highlights the need for improved monitoring programmes with sufficient sample size and stressor gradient coverage. Our findings emphasize the importance of adaptive management, as stress reduction measures or further ecosystem degradation may change multiple stressor-effect relationships, which will then require associated changes in management strategies.

Substrate size modifies stream grazer-biofilm interactions in the presence of invertivorous fish.

When herbivore abundance is controlled by predators there may be an indirect positive effect on primary producers due to reduced grazing pressure, but the potential of predation refuges to modify such trophic cascades has rarely been studied. By experimentally manipulating substrate particle size and fish predation regime, we assessed the outcome of invertebrate grazer-biofilm interactions in streams. Locations at the center of larger substrate particles were predicted to pose a higher predation risk, and therefore be subjected to a lower grazing pressure. In our 52-day experiment in a New Zealand stream, small-sized substrates (terracotta tiles) remained virtually free of periphyton across their entire upper surface, whereas a thick periphyton mat was formed across large tiles with only edges remaining free. In channels containing fish (either native Galaxias vulgaris or exotic Salmo trutta), grazing on tiles was lower than in the absence of fish. A preference for grazing near to the edge of tiles was clearest in fish channels but was also evident even in the absence of fish, probably reflecting fish presence and/or fish kairomones in the stream from where the colonizing invertebrates had been derived. Total grazer density was similar across treatments with or without fish, suggesting that our results can be explained mostly by changes in the behavior of grazers. We suggest that refuge availability, interacting with grazer predator-avoidance behavior, may produce a context-dependent patchwork of trophic cascades in streams and other ecosystems.

Warming and imidacloprid pulses determine macroinvertebrate community dynamics in experimental streams.

Sustainable management of freshwater and pesticide use is essential for mitigating the impacts of intensive agriculture in the context of a changing climate. To better understand how climate change will affect the vulnerability of freshwater ecosystems to chemical pollutants, more empirical evidence is needed on the combined effects of climatic and chemical stressors in environmentally realistic conditions. Our experiment provides the first empirical evaluation of stream macroinvertebrate community dynamics in response to one of the world's most widely used insecticides, imidacloprid, and increased water temperature. In a 7-week streamside experiment using 128 flow-through circular mesocosms, we investigated the effects of pulsed imidacloprid exposure (four environmentally relevant levels between 0 and 4.6 µg/L) and raised water temperature (ambient, 3°C above) on invertebrate communities representative of fast- and slow-flowing microhabitats. Invertebrate drift and insect emergence were monitored during three pesticide pulses (10 days apart), and benthic invertebrate communities were sampled after 24 days of heating and pesticide manipulations. All three manipulated factors strongly affected drift community composition. The first imidacloprid pulse and increased temperature had a greater impact on communities in fast-flowing mesocosms, which contained more pollution-sensitive EPT taxa (mayflies, stoneflies and caddisflies). Heating and imidacloprid caused increased emigration by drift, weak reductions in emergence, and negatively affected the benthic community. The combined effect of stressor manipulations and a 10-day natural heatwave drastically reduced relative abundances of EPT and insects overall and caused a shift to oligochaete-, crustacean- and gastropod-dominated communities. Contrary to our hypothesis, the very high yet realistic water temperatures reached in our experiment meant the negative effects of imidacloprid were clearest at ambient temperatures and fast flow. These findings demonstrate the potential combined impacts of imidacloprid contamination and heatwaves on freshwater invertebrate communities under future climate scenarios and highlight the need for more countries to take regulatory action to control neonicotinoid use.

Impacts of multiple anthropogenic stressors on stream macroinvertebrate community composition and functional diversity.

Ensuring the provision of essential ecosystem services in systems affected by multiple stressors is a key challenge for theoretical and applied ecology. Trait-based approaches have increasingly been used in multiple-stressor research in freshwaters because they potentially provide a powerful method to explore the mechanisms underlying changes in populations and communities. Individual benthic macroinvertebrate traits associated with mobility, life history, morphology, and feeding habits are often used to determine how environmental drivers structure stream communities. However, to date multiple-stressor research on stream invertebrates has focused more on taxonomic than on functional metrics. We conducted a fully crossed, 4-factor experiment in 64 stream mesocosms fed by a pristine montane stream (21 days of colonization, 21 days of manipulations) and investigated the effects of nutrient enrichment, flow velocity reduction and sedimentation on invertebrate community, taxon, functional diversity and trait variables after 2 and 3 weeks of stressor exposure. 89% of the community structure metrics, 59% of the common taxa, 50% of functional diversity metrics, and 79% of functional traits responded to at least one stressor each. Deposited fine sediment and flow velocity reduction had the strongest impacts, affecting invertebrate abundances and diversity, and their effects translated into a reduction of functional redundancy. Stressor effects often varied between sampling occasions, further complicating the prediction of multiple-stressor effects on communities. Overall, our study suggests that future research combining community, trait, and functional diversity assessments can improve our understanding of multiple-stressor effects and their interactions in running waters.

Time-cumulative effects of neonicotinoid exposure, heatwaves and food limitation on stream mayfly nymphs: A multiple-stressor experiment.

The global intensification of agriculture has resulted in pesticides playing an increasingly important role as anthropogenic stressors and drivers of environmental change. There is also a growing need to determine if other environmental stressors, especially those predicted to worsen with climate change, interact with pesticides to alter their effects on non-target biota. Two such stressors are increased extreme temperature events and periods of food limitation. This study is the first to investigate the combined effects of the world's most widely used insecticide, imidacloprid, with heatwaves and food limitation on a freshwater animal. A 6-week, full-factorial laboratory experiment with Deleatidium spp. mayfly nymphs was performed to investigate the potential for direct and delayed interactive effects of simulated heatwaves and starvation with chronic exposure to a field-realistic concentration of imidacloprid (0.4 µg/L). The experiment included two 6-day simulated heatwaves, one during a starvation period prior to imidacloprid addition, and one during the first 6 days of imidacloprid exposure. The simulated heatwaves alone caused such drastic negative effects on Deleatidium survival and mobility that mainly antagonistic interactions were observed with the other stressors, though delayed synergisms between imidacloprid and the second heatwave also affected mayfly mobility. Time-cumulative toxicity of imidacloprid was evident, with imidacloprid first affecting mayfly mobility after 12 days but eventually causing the strongest effects of all manipulated stressors. However, lethal effects of imidacloprid could only be detected in the absence of heatwaves and starvation, possibly as a result of selection for stronger individuals due to prior exposure to these stressors. Our findings demonstrate that heatwaves of increasing severity will critically affect sensitive freshwater organisms such as mayflies, and that the impacts of widespread pesticide use on freshwater ecosystems under global climate change cannot be ignored.

Imidacloprid dominates the combined toxicities of neonicotinoid mixtures to stream mayfly nymphs.

Contamination of the environment with toxic chemicals such as pesticides has become a global problem. Understanding the role of chemical contaminants as stressors in ecological systems is therefore an important research need in the 21st century. In surface freshwaters, mixtures of neonicotinoid insecticides are being detected around the world as more monitoring data become available. Combinations of imidacloprid, clothianidin and thiamethoxam are commonly found, but studies testing their combined toxicities to freshwater invertebrates are rare. Taking a multiple-stressor approach, we employed a full-factorial design to investigate the individual and combined chronic toxicities of these three neonicotinoids in a 28-day laboratory experiment using Deleatidium spp. mayfly nymphs. Imidacloprid (1.2 µg/L achieved concentration) reduced mayfly survival (by 50% on Day 28) and mobility (~100%) more than clothianidin (1.1 µg/L, affecting about 25% of individuals across the responses measured) and thiamethoxam (2.9 µg/L, affecting 12%). Imidacloprid interacted with the other two neonicotinoids to cause a greater-than-additive negative effect when combined until 25 days of exposure, after which the strong negative overall effects of imidacloprid prevented these interactions from being observed. Our findings represent a novel contribution to multiple-stressor research by demonstrating the combined effects of chronic exposure to environmentally relevant neonicotinoid concentrations on an ecologically important stream insect taxon. These results emphasise the higher toxicity of imidacloprid to non-target freshwater insects compared to clothianidin and thiamethoxam, implying that stricter regulation to control the use of imidacloprid may need to be prioritised to protect vulnerable aquatic insect populations that provide key links to terrestrial food webs. Finally, our study provides an ecological, multiple-stressor comparison for related ecotoxicological investigations indicating neonicotinoid mixtures can deviate from additive toxicity.

Fine sediment and flow velocity impact bacterial community and functional profile more than nutrient enrichment.

Freshwater ecosystems face many simultaneous pressures due to human activities. Consequently, there has been a rapid loss of freshwater biodiversity and an increase in biomonitoring programs. Our study assessed the potential of benthic stream bacterial communities as indicators of multiple-stressor impacts associated with urbanization and agricultural intensification. We conducted a fully crossed four-factor experiment in 64 flow-through mesocosms fed by a pristine montane stream (21 d of colonization, 21 d of manipulations) and investigated the effects of nutrient enrichment, flow-velocity reduction and added fine sediment after 2 and 3 weeks of stressor exposure. We used high-throughput sequencing and metabarcoding techniques (16S rRNA genes), as well as curated biological databases (METAGENassit, MetaCyc), to identify changes in bacterial relative abundances and predicted metabolic functional profile. Sediment addition and flow-velocity reduction were the most pervasive stressors. They both increased α-diversity and had strong taxon-specific effects on community composition and predicted functions. Sediment and flow velocity also interacted frequently, with 88% of all bacterial response variables showing two-way interactions and 33% showing three-way interactions including nutrient enrichment. Changes in relative abundances of common taxa were associated with shifts in dominant predicted functions, which can be extrapolated to underlaying stream-wide mechanisms such as carbon use and bacterial energy production pathways. Observed changes were largely stable over time and occurred after just 2 weeks of exposure, demonstrating that bacterial communities can be well-suited for early detection of multiple stressors. Overall, added sediment and reduced flow velocity impacted both bacterial community structure and predicted function more than nutrient enrichment. In future research and stream management, a holistic approach to studying multiple-stressor impacts should include multiple trophic levels with their functional responses, to enhance our mechanistic understanding of complex stressor effects and promote establishment of more efficient biomonitoring programs.

Towards a unified study of multiple stressors: divisions and common goals across research disciplines.

Anthropogenic environmental changes, or 'stressors', increasingly threaten biodiversity and ecosystem functioning worldwide. Multiple-stressor research is a rapidly expanding field of science that seeks to understand and ultimately predict the interactions between stressors. Reviews and meta-analyses of the primary scientific literature have largely been specific to either freshwater, marine or terrestrial ecology, or ecotoxicology. In this cross-disciplinary study, we review the state of knowledge within and among these disciplines to highlight commonality and division in multiple-stressor research. Our review goes beyond a description of previous research by using quantitative bibliometric analysis to identify the division between disciplines and link previously disconnected research communities. Towards a unified research framework, we discuss the shared goal of increased realism through both ecological and temporal complexity, with the overarching aim of improving predictive power. In a rapidly changing world, advancing our understanding of the cumulative ecological impacts of multiple stressors is critical for biodiversity conservation and ecosystem management. Identifying and overcoming the barriers to interdisciplinary knowledge exchange is necessary in rising to this challenge. Division between ecosystem types and disciplines is largely a human creation. Species and stressors cross these borders and so should the scientists who study them.

Agricultural Land-Use Legacy, The Invasive Alga Didymosphenia geminata and Invertebrate Communities in Upland Streams with Natural Flow Regimes.

The integrity of freshwater ecosystems worldwide is under threat from agriculture and invasive species. Past agricultural activity can have persistent effects on aquatic diversity even decades after restoration, and the spread of invasive species is increasingly difficult to prevent due to globalisation. In the South Island of New Zealand, the invasive diatom Didymosphenia geminata (Didymo) causes nuisance blooms in streams. The impact of Didymo on stream invertebrate communities in upland streams with natural flow regimes remains poorly understood. We investigated the relationships between legacy effects of agriculture, Didymo and benthic invertebrate communities at 55 stream sites in Mahu Whenua, a 530 km2 conservation area comprising four former New Zealand high-country farms. The farms were destocked of sheep 4-9 years before stream sampling started. Kick-netting was used to collect macroinvertebrates from 7-23 streams within each farm to provide a land-use legacy gradient. Moreover, samples from 16 sites with clearly visible Didymo mats covering most of the stream bed (indicating high biomass and a dominant role in the biofilm) were compared with 39 sites without such Didymo mats. Total invertebrate taxon richness and EPT richness (taxon richness of larval mayflies, stoneflies and caddisflies) were lower in the stream catchments destocked most recently. When Didymo was present, relative EPT abundance was lower than when Didymo was absent, and Deleatidium mayflies decreased whereas midges and oligochaetes increased. These results highlight the need to look at past land-use practices when restoring high-country streams after agricultural impacts. They also show that Didymo can have negative effects on invertebrate communities in upland streams with natural flow regimes, a stream type previously overlooked in studies on this invasive diatom.

Multiple stressors and stream macroinvertebrate community dynamics: Interactions between fine sediment grain size and flow velocity.

Agricultural development has resulted in the degradation of freshwater ecosystems worldwide. Two key stressors impacting streams and rivers draining agricultural catchments are deposited fine sediment (e.g. due to erosion) and reduced flows (e.g. due to water abstraction, dams, or climate change). Past studies have identified fine sediment as a 'master stressor' in streams, but the effects of different sediment grain sizes in combination with reduced flow velocity are poorly understood. We manipulated deposited fine sediment (no added sediment; silt: 0-0.125 mm; fine sand: 0.125-0.250 mm; coarse sand: 1-2 mm) and flow velocity (fast: 26.5 cm/s; medium: 13.9 cm/s; slow: 0.0 cm/s) simultaneously in 60 outdoor stream mesocosms. We determined the individual and combined effects of these stressors on the benthic, drifting, and emerging stream macroinvertebrate communities. Both fine sediment and reduced flow velocity had pervasive detrimental impacts on stream invertebrate communities. Negative effects of sediment were worse at the smaller two grain sizes for some responses (abundance of Chironomidae, Copepoda, Psilochorema spp.); however, for several sediment-sensitive common taxa or community-level invertebrate metrics, effects were negative regardless of grain size. Although their combined effects were mainly additive, sediment impacts were worsened by reduced flow velocities in several cases. Our findings imply that (a) especially for sediment-sensitive species, all fine sediment <2 mm has profound negative effects, (b) sediment grain size matters for some invertebrate taxa, where severity of impacts increased as particle size decreased, and (c) negative effects of sedimentation can become worse when combined with reduced flow velocity.

Anthropogenic stressors affect fungal more than bacterial communities in decaying leaf litter: A stream mesocosm experiment.

Despite the progress made in environmental microbiology techniques and knowledge, the succession and functional changes of the microbial community under multiple stressors are still poorly understood. This is a substantial knowledge gap as microbial communities regulate the biogeochemistry of stream ecosystems. Our study assessed the structural and temporal changes in stream fungal and bacterial communities associated with decomposing leaf litter under a multiple-stressor scenario. We conducted a fully crossed 4-factor experiment in 64 flow-through mesocosms fed by a pristine montane stream (21 days of colonisation, 21 days of manipulations) and investigated the effects of nutrient enrichment, flow velocity reduction and sedimentation after 2 and 3 weeks of stressor exposure. We used high-throughput sequencing and metabarcoding techniques (16S and 18S rRNA genes) to identify changes in microbial community composition. Our results indicate that (1) shifts in relative abundances of the pre-existing terrestrial microbial community, rather than changes in community identity, drove the observed responses to stressors; (2) changes in relative abundances within the microbial community paralleled decomposition rate patterns with time; (3) both fungal and bacterial communities had a certain resistance to stressors, as indicated by relatively minor changes in alpha diversity or multivariate community structure; (4) overall, stressor interactions were more common than stressor main effects when affecting microbial diversity metrics or abundant individual genera; and (5) stressor effects on microbes often changed from 2 weeks to 3 weeks of stressor exposure, with several response patterns being reversed. Our study suggests that future research should focus more on understanding the temporal dynamics of fungal and bacterial communities and how they relate to ecosystem processes to advance our understanding of the mechanisms associated with multiple-stressor interactions.

Multiple-stressor effects of dicyandiamide (DCD) and agricultural stressors on trait-based responses of stream benthic algal communities.

Agricultural practices often result in multiple stressors affecting stream ecosystems, and interacting stressors complicate environmental assessment and management of impacted streams. The nitrification inhibitor dicyandiamide (DCD) is used for nitrogen management on farmland. Effects of leached DCD on stream ecosystems are still largely unstudied, even though it could be relevant as a stressor on its own or in combination with other agricultural stressors. We conducted two experiments in 128 outdoor stream-fed mesocosms to assess stressor effects on biomass, cell density, taxon richness, evenness and functional trait composition of benthic algal communities. First, we examined responses to a wide DCD gradient (eight concentrations, 0-31 mg L-1) and two additional stressors, deposited fine sediment (none, high) and nutrient enrichment (ambient, enriched). Second, we determined algal responses to four stressors: DCD, sediment, nutrients, and reduced flow velocity. Here DCD treatments included controls, constant application (1.4 mg L-1) and two pulsed treatments mimicking concentration patterns in real streams (peaks 3.5 mg L-1, 2.2 mg L-1). Sediment and nutrient enrichment were influential stressors in both experiments, with fine sediment having the most pervasive effects. In Experiment 2, reduced flow velocity had pervasive effects and stressor interactions were mainly restricted to two-way interactions. DCD had few, weak stressor main effects, especially at field-realistic concentrations (Experiment 2). At the highest concentrations in Experiment 1 (above levels observed in real streams), DCD effects were still rare but some significant stressor interactions occurred. Analyses of functional traits were helpful in identifying potential mechanisms driving changes in densities and community composition. These findings suggest that, while DCD on its own may be a minor stressor, it could have adverse effects on algal communities already exposed to other stressors, a scenario common in agricultural streams.

Current-use pesticides in New Zealand streams: Comparing results from grab samples and three types of passive samplers.

New Zealand uses more than a ton of pesticides each year; many of these are mobile, relatively persistent, and can make their way into waterways. While considerable effort goes into monitoring nutrients in agricultural streams and programs exist to monitor pesticides in groundwater, very little is known about pesticide detection frequencies, concentrations, or their potential impacts in New Zealand streams. We used the 'Polar Organic Chemical Integrative Sampler' (POCIS) approach and grab water sampling to survey pesticide concentrations in 36 agricultural streams in Waikato, Canterbury, Otago and Southland during a period of stable stream flows in Austral summer 2017/18. We employed a new approach for calculating site-specific POCIS sampling rates. We also tested two novel passive samplers designed to reduce the effects of hydrodynamic conditions on sampling rates: the 'Organic-Diffusive Gradients in Thin Films' (o-DGT) aquatic passive sampler and microporous polyethylene tubes (MPTs) filled with Strata-X sorbent. Multiple pesticides were found at most sites; two or more were detected at 78% of sites, three or more at 69% of sites, and four or more at 39% of sites. Chlorpyrifos concentrations were the highest, with a maximum concentration of 180 ng/L. Concentrations of the other pesticides were generally below 20 ng/L. Mean concentrations of individual pesticides were not correlated with in-stream nutrient concentrations. The majority of pesticides were detected most frequently in POCIS, presumably due to its higher sampling rate and the relatively low concentrations of these pesticides. In contrast, chlorpyrifos was most frequently detected in grab samples. Chlorpyrifos concentrations at two sites were above the 21-day chronic 'No Observable Effect Concentration' (NOEC) values for fish and another two sites had concentrations greater than 50% of the NOEC. Otherwise, concentrations were well-below NOEC values, but close to the New Zealand Environmental Exposure Limits in several cases.

Chronic Toxicities of Neonicotinoids to Nymphs of the Common New Zealand Mayfly Deleatidium spp.

Neonicotinoid insecticides have been shown to have high chronic toxicity relative to acute toxicity, and therefore short-term toxicity tests ≤96 h in duration may underestimate their environmental risks. Among nontarget aquatic invertebrates, insects of the orders Diptera and Ephemeroptera have been found to be the most sensitive to neonicotinoids. To undertake a more accurate assessment of the risks posed by neonicotinoids to freshwater ecosystems, more data are needed from long-term tests employing the most sensitive taxa. Using nymphs of the common New Zealand mayfly genus Deleatidium spp., we performed 28-d static-renewal exposures with the widely used neonicotinoids imidacloprid, clothianidin, and thiamethoxam. We monitored survival, immobility, impairment, and mayfly moulting propensity at varying time points throughout the experiment. Imidacloprid and clothianidin exerted strong chronic toxicity effects on Deleatidium nymphs, with 28-d median lethal concentrations (LC50s) of 0.28 and 1.36 µg/L, respectively, whereas thiamethoxam was the least toxic, with a 28-d LC50 > 4 µg/L (highest concentration tested). Mayfly moulting propensity was also negatively affected by clothianidin (during 3 of 4 wk), imidacloprid (2 of 4 wk), and thiamethoxam (1 of 4 wk). Comparisons with published neonicotinoid chronic toxicity data for other mayfly taxa and larvae of the midge genus Chironomus showed similar sensitivities for mayflies and midges, suggesting that experiments using these taxa provide reliable assessments of the threats of neonicotinoids to the most vulnerable freshwater species. Environ Toxicol Chem 2019;38:2459-2471. © 2019 SETAC.

Chronic nutrient inputs affect stream macroinvertebrate communities more than acute inputs: An experiment manipulating phosphorus, nitrogen and sediment.

Freshwaters worldwide are affected by multiple stressors. Timing of inputs and pathways of delivery can influence the impact stressors have on freshwater communities. In particular, effects of point versus diffuse nutrient inputs on stream macroinvertebrates are poorly understood. Point-source inputs tend to pose a chronic problem, whereas diffuse inputs tend to be acute with short concentration spikes. We manipulated three key agricultural stressors, phosphorus (ambient, chronic, acute), nitrogen (ambient, chronic, acute) and fine sediment (ambient, high), in 112 stream mesocosms (26 days colonisation, 18 days of manipulations) and determined the individual and combined effects of these stressors on stream macroinvertebrate communities (benthos and drift). Chronic nutrient treatments continuously received high concentrations of P and/or N. Acute channels received the same continuous enrichment, but concentrations were doubled during two 3-hour periods (day 6, day 13) to simulate acute nutrient inputs during rainstorms. Sediment was the most pervasive stressor in the benthos, reducing total macroinvertebrate abundance and richness, EPT (mayflies, stoneflies, caddisflies) abundance and richness. By contrast, N or P enrichment did not affect any of the six studied community-level metrics. In the drift assemblage, enrichment effects became more prevalent the longer the experiment went on. Sediment was the dominant driver of drift responses at the beginning of the experiment. After the first acute nutrient pulse, sediment remained the most influential stressor but its effects started to fade. After the second pulse, N became the dominant stressor. In general, impacts of either N or P on the drift were due to chronic exposure, with acute nutrient pulses having no additional effects. Overall, our findings imply that cost-effective management should focus on mitigating sediment inputs first and tackle chronic nutrient inputs second. Freshwater managers should also take into account the length of exposure to high nutrient concentrations, rather than merely the concentrations themselves.

Biodiversity of leaf litter fungi in streams along a latitudinal gradient.

Global patterns of biodiversity have emerged for soil microorganisms, plants and animals, and the extraordinary significance of microbial functions in ecosystems is also well established. Virtually unknown, however, are large-scale patterns of microbial diversity in freshwaters, although these aquatic ecosystems are hotspots of biodiversity and biogeochemical processes. Here we report on the first large-scale study of biodiversity of leaf-litter fungi in streams along a latitudinal gradient unravelled by Illumina sequencing. The study is based on fungal communities colonizing standardized plant litter in 19 globally distributed stream locations between 69°N and 44°S. Fungal richness suggests a hump-shaped distribution along the latitudinal gradient. Strikingly, community composition of fungi was more clearly related to thermal preferences than to biogeography. Our results suggest that identifying differences in key environmental drivers, such as temperature, among taxa and ecosystem types is critical to unravel the global patterns of aquatic fungal diversity.