Inconsistent browning of northeastern U.S. lakes despite increased precipitation and recovery from acidification

Groundwater inputs could be a significant but often overlooked source of phosphorus in lake ecosystems

Freshwater lakes are severely threatened, due largely to excess inputs of nutrients and other contaminants. Phosphorus (P) is receiving renewed attention due to recent increases in toxic cyanobacteria blooms in lakes worldwide. We investigated groundwater seepage for its role in P loading dynamics at Oneida Lake, New York, USA-one of the most well-studied lakes globally. P loading was measured at representative sites along the 88 km shoreline over three summers by directly measuring groundwater flow using seepage meters and porewater samplers. Groundwater seepage was a continuous and significant source of dissolved P over the summer months, comparable to tributary sources to the lake during that time. This constant input has enriched the concentrations of P in the nearshore surface waters, significantly above levels in the pelagic zone. Pore Total Phosphorus (TP) concentrations and loads reached extremely high values (up to 100 mg/L), with inorganic P representing only ~ 10% of TP per site. Groundwater seepage flows and P loadings were highly variable across space and time, partially explained by adjacent land uses and precipitation. Our research concludes that groundwater seepage is a significant, but overlooked, source of dissolved P and a crucial factor driving summer primary production at Oneida Lake, and likely other temperate lakes.

The response of aquatic ecosystems to the interactive effects of stratospheric ozone depletion, UV radiation, and climate change

Variations in stratospheric ozone and changes in the aquatic environment by climate change and human activity are modifying the exposure of aquatic ecosystems to UV radiation. These shifts in exposure have consequences for the distributions of species, biogeochemical cycles, and services provided by aquatic ecosystems. This Quadrennial Assessment presents the latest knowledge on the multi-faceted interactions between the effects of UV irradiation and climate change, and other anthropogenic activities, and how these conditions are changing aquatic ecosystems. Climate change results in variations in the depth of mixing, the thickness of ice cover, the duration of ice-free conditions and inputs of dissolved organic matter, all of which can either increase or decrease exposure to UV radiation. Anthropogenic activities release oil, UV filters in sunscreens, and microplastics into the aquatic environment that are then modified by UV radiation, frequently amplifying adverse effects on aquatic organisms and their environments. The impacts of these changes in combination with factors such as warming and ocean acidification are considered for aquatic micro-organisms, macroalgae, plants, and animals (floating, swimming, and attached). Minimising the disruptive consequences of these effects on critical services provided by the world's rivers, lakes and oceans (freshwater supply, recreation, transport, and food security) will not only require continued adherence to the Montreal Protocol but also a wider inclusion of solar UV radiation and its effects in studies and/or models of aquatic ecosystems under conditions of the future global climate.

A review of long-term change in surface water natural organic matter concentration in the northern hemisphere and the implications for drinking water treatment

Reduced atmospheric acid deposition has given rise to recovery from acidification - defined as increasing pH, acid neutralization capacity (ANC), or alkalinity in surface waters. Strong evidence of recovery has been reported across North America and Europe, driving chemical responses. The primary chemical responses identified in this review were increasing concentration and changing character of natural organic matter (NOM) towards predominantly hydrophobic nature. The concentration of NOM also influenced trace metal cycling as many browning surface waters also reported increases in Fe and Al. Further, climate change and other factors (e.g., changing land use) act in concert with reductions in atmospheric deposition to contribute to widespread browning and will have a more pronounced effect as deposition stabilizes. The observed water quality trends have presented challenges for drinking water treatment (e.g., increased chemical dosing, poor filter operations, formation of disinfection by-products) and many facilities may be under designed as a result. This comprehensive review has identified key research areas to be addressed, including 1) a need for comprehensive monitoring programs (e.g., larger timescales; consistency in measurements) to assess climate change impacts on recovery responses and NOM dynamics, and 2) a better understanding of drinking water treatment vulnerabilities and the transition towards robust treatment technologies and solutions that can adapt to climate change and other drivers of changing water quality.

Brownification on hold: What traditional analyses miss in extended surface water records

Widespread increases in organic matter (OM) content of surface waters, as measured by color and organic carbon (OC), are a major issue for aquatic ecosystems. Long-term monitoring programs revealed the issue of "brownification", with climate change, land cover changes and recovery from acidification all suspected to be major drivers or contributing factors. While many studies have focused on the impact and drivers, fewer have followed up on whether brownification is continuing. As time-series of OM data lengthen, conventional data-analysis approaches miss important information on when changes occur. To better identify temporal OM patterns during three decades (1990-2020) of systematic monitoring, we used generalized additive models to analyze 164 time-series from watercourses located across Sweden. Increases in OC that were widespread during 1990-2010 ceased a decade ago, and most color increases ceased 20 years ago. These findings highlight the need to reassess the understanding of brownification's spatial and temporal extent, as well as the tools used to analyze lengthening time series.