ABSTRACT Terrestrial studies and pelagic studies in lakes generally report positive biodiversity–ecosystem functioning relationships, where more diverse communities perform better. In large rivers, strong environmental constraints constantly filter plankton communities, promoting species sorting and the dominance of well‐adapted taxa that may alter the mechanisms linking biodiversity to ecosystem functioning. Here, we evaluated the link between environmental filtering and zooplankton community composition, and then tested whether ecosystem functioning is primarily associated with complementarity or selection effects associated with dominant species in the middle section of the River Danube. We quantified resource use efficiency as an ecosystem functioning measure using the ratio of zooplankton to phytoplankton carbon biomass, alongside taxonomic (species richness and evenness) and functional (functional richness and evenness) diversity measures as predictors. Our analysis relied on a multi‐year high‐frequency dataset, covering a long environmental gradient across contrasting hydrological years. Our results demonstrated that local environmental variables, linked primarily to discharge, were associated with constrained zooplankton community composition based on both taxonomic and functional data. Assessing diversity measures individually, resource use efficiency scaled positively with zooplankton taxonomic and functional richness; meanwhile, evenness only mattered in Rotifera. Concurrently, a negative relationship with taxonomic evenness revealed that pronounced community dominance, associated with biomass peaks of a few highly competitive opportunistic taxa, maximised community‐level resource use via mass ratio effect. Considering all diversity measures jointly, the best‐performing models combined trait richness and taxonomic evenness, providing a better explanation of ecosystem functioning than either predictor alone. Taxonomic and functional trait richness were almost perfectly correlated, explaining why functional richness retained explanatory power whereas traditional species richness did not in combined models. Since dominance‐driven effects can only act on the available species pool, we argue that high functional richness helps maintain complementary ecological strategies that maximise resource use under varying environmental conditions, while environmental filtering promotes the dominance of the highly competitive opportunistic taxa, that is, Rotifera in large rivers. Our findings indicate that community dominance does not replace biodiversity effects; rather, environmental filtering operates upon a functionally diverse species pool, making functional richness and community dominance complementary qualities influencing ecosystem functioning in large river zooplankton.
ABSTRACT High‐mountain lakes are among the most sensitive ecosystems to global environmental change. Their remote locations and low anthropogenic disturbance make them ideal sentinels for detecting subtle, long‐term ecological responses. This study investigated decadal‐scale changes in aquatic macrophyte communities and water chemistry across a network of remote lakes in the Pyrenees. Our aim was to assess the extent to which regional macrophyte assemblages have shifted over the past 36 years and to identify the environmental variables—both global and local—associated with these changes. We surveyed aquatic macrophyte communities in 30 high‐altitude lakes across the Pyrenees, southwestern Europe, comparing data collected in 1987–1988 with resurveys conducted between 2017 and 2023. Macrophyte assemblages were assessed through visual transects in clear, oligotrophic waters. Water chemistry was analysed from shoreline samples in 1987–1988, 2011 and 2023, focusing on conductivity, alkalinity, major ions, phosphorus and dissolved inorganic nitrogen. Temporal β‐diversity metrics were used to quantify community turnover and nestedness, with patterns interpreted in the context of long‐term climate trends, atmospheric deposition and lake‐specific characteristics. While total macrophyte richness across the region remained relatively stable, we observed marked shifts in community composition. Bicarbonate‐using elodeid species (e.g., Potamogeton spp., Myriophyllum alterniflorum ) became more prevalent, particularly in lakes where alkalinity exceeded 150 μeq/L or conductivity surpassed 20 μS/cm over the study period. In contrast, mosses and floating‐leaved taxa declined, particularly in ultra‐oligotrophic systems. These biotic changes paralleled increases in alkalinity, conductivity and major ion concentrations, suggesting intensified rock weathering and dust deposition. Dissolved inorganic nitrogen, by contrast, declined over time, reflecting unbalanced nitrogen (N) and phosphorus (P) deposition in favour of P. Our findings reveal that aquatic macrophyte communities in Pyrenean high‐mountain lakes have undergone significant compositional turnover over the past three decades, even in the absence of major local disturbances. These changes are best explained by regional‐scale biogeochemical shifts linked to climate change and atmospheric inputs, rather than by local stressors such as fish introductions. Chemical thresholds for alkalinity and conductivity appear to mark tipping points in macrophyte assemblage structure, emphasizing the vulnerability of ultra‐oligotrophic lakes to even modest changes in water chemistry. Our results underscore the utility of macrophyte communities as ecological indicators and highlight the need for continued monitoring of high‐altitude lake systems worldwide, many of which face similar pressures from climate change, atmospheric deposition and biogeochemical alterations.
ABSTRACT Aquatic insect emergence is a critical life history stage whose magnitude and phenology respond to multiple environmental factors. Here, we report the first estimates of spatial and temporal variation in aquatic insect emergence from oligotrophic streams of the Hubbard Brook Experimental Forest (HBEF, New Hampshire, USA) over the period 2018–2024. This record is one of very few multi‐site, multi‐year aquatic insect emergence time series. We measured insect emergence from seven gauged, headwater streams and the mainstem of Hubbard Brook, a fifth‐order stream into which all of the headwater streams drain. Emergence traps were deployed and collected weekly during the snow‐free months (April—November). These emergence samples complement long‐term weekly stream chemistry monitoring (since 1963) and were coincident with high‐frequency water temperature monitoring. Insects were collected on sticky traps hung above the streams and identified to order level. Here, we report the emergence patterns for two streams over 7 years, five streams for 3 years, and one stream for 2 years. Total annual emergence counts of aquatic insects varied from the lowest count of 1887 individuals from the Hubbard Brook mainstem in 2018 to 22,628 individuals from headwater stream W5 in 2022. Emergence was dominated by aquatic Diptera, and despite the large variation in emergence magnitude, the timing of peak emergence was consistently in mid to late May (Julian day = 140 ± 9). Our data document highly variable secondary productivity from these streams despite their low nutrient availability and strong seasonality in peak emergence timing. Collection of emergence samples is ongoing which—when coupled with long‐term HBEF streamflow and chemistry data—represents an opportunity to examine the effects of increasingly intense precipitation events and resulting high streamflow, which may scour the streambed, as well as the effects of recovery from acid rain, which is increasing stream pH. Understanding the consequences of changing stream temperature, flow, and chemistry on the magnitude and timing of aquatic insect emergence is critical for predicting effects on their populations and life histories.
ABSTRACT Trophic transfer efficiency (TTE) is commonly quantified from carbon fluxes, implicitly assuming that primary production is uniformly available to consumers. This assumption fails in many eutrophic lakes, where high phytoplankton biomass and carbon production coexist with weak secondary production, a paradox that carbon‐centred frameworks have recognized but not mechanistically resolved. Here, we propose that this mismatch may arise because trophic transfer is constrained not simply by production quantity, but by trophic usability. We applied a dual‐currency framework to examine phytoplankton‐to‐zooplankton transfer in more and less eutrophic regions of two subtropical lakes. Transfer was quantified using bulk carbon and ω3 long‐chain polyunsaturated fatty acids (ω3 LC‐PUFA), together with information on plankton community composition and inferred trophic‐network structure. Our results show that the more eutrophic regions had higher phytoplankton carbon production and stronger cyanobacterial dominance, whereas the less eutrophic regions had higher phytoplankton ω3 LC‐PUFA production and greater contributions of diatoms and cryptophytes. These contrasts coincided with higher zooplankton carbon and ω3 LC‐PUFA production and higher TTE in the less eutrophic regions. ω3 LC‐PUFA‐based TTE was generally higher than carbon‐based TTE, although the difference was not statistically significant in every regional comparison. This metric is therefore best interpreted as a quality‐weighted transfer and retention measure reflecting both trophic routing and selective biochemical conservation. Exploratory networks based on observed taxa contained more inferred links in the less eutrophic regions, consistent with regional differences in plankton community structure. These findings support the view that trophic transfer in eutrophic lakes is associated with the quantity and biochemical quality of primary production, plankton community composition, and selective retention of essential molecules. Although based on a two‐lake, single‐season correlative comparison, the dual‐currency framework provides a complementary approach for evaluating how eutrophication‐related changes in basal resources are linked to consumer production and trophic transfer.
ABSTRACT Freshwater ecosystems are increasingly threatened by human activities and climate change, emphasizing the need for effective habitat restoration supported by long‐term monitoring. We examined fish species responses to instream restoration in six boreal streams in northern Finland over a 24‐year period (1999–2022) using a Before–After–Control–Impact (BACI) design, including 3 years before and 21 years after restoration. Each stream contained three reaches: one restored with large woody debris and stones (LWD + s), one with added stones only and one left as a control. Using generalized linear mixed models, we assessed the effects of restoration type, water temperature and discharge on total fish density and biomass as well as on key species, brown trout ( Salmo trutta ), burbot ( Lota lota ) and European bullhead ( Cottus gobio ). Overall fish abundance and biomass declined over time, including in control reaches, suggesting a shifting baseline of ecological conditions. However, LWD + s‐restored reaches showed weaker long‐term declines in fish density and biomass than control reaches, indicating that wood additions buffered fish populations against long‐term environmental deterioration. Brown trout showed more positive long‐term responses in reaches containing LWD + s, whereas burbot responses were more strongly associated with stone additions, reflecting species‐specific habitat preferences. European bullhead was not affected by restoration. Rising water temperatures and variable flows were associated with reduced fish densities, consistent with regional climatic trends. Our results demonstrate that while local restoration can mitigate habitat degradation, its success is constrained by broader hydrological and thermal variability. Effective river management in boreal regions may require integrating catchment‐level water retention measures with instream habitat enhancement, supported by sustained long‐term BACI‐based monitoring to capture delayed ecological responses.
