We evaluated trends in dissolved organic carbon (DOC) and associated changes in water transparency and epilimnion thickness to better understand the implications of regional increases in DOC concentration in lakes. Long‐term monitoring of a suite of physical, chemical, and biological data from six to 12 lakes in Acadia National Park in Maine was paired with high‐frequency sensor monitoring of one lake as a model system. Water transparency declined across study sites since 1995 as DOC increased and chlorophyll remained stable, suggesting that this was not a signal of increased eutrophication. As clarity declined, some lakes experienced reduced epilimnion thickness. The degree to which transparency changed across the lakes was dependent on DOC concentration, with a larger decline in transparency occurring in clear water lakes (−0.3 m yr−1) than brown water lakes (−0.1 m yr−1). DOC concentration was an important explanatory variable for reduced epilimnion thickness in short‐term sensor measurements. A regional decline in water transparency across all lakes and reduction in epilimnion thickness in a limited number of systems appeared to be acting as a sentinel for changes in atmospheric deposition and regional weather that modified the delivery of DOC from the watershed.
Recent national and international policies to control Hg sources to the environment highlight the importance of biomonitoring and documenting spatial and temporal patterns and changes in the magnitude of Hg concentrations of abiotic and biotic compartments. However, there are relatively few locations where comprehensive assessments of spatial and temporal data on the patterns of Hg in abiotic and biotic compartments have been conducted. One of the ecosystems where Hg fate, transport, and bioaccumulation have been studied extensively in multi-media over several decades is Acadia National Park. Hg data collected between 1993 and 2021 from abiotic and biotic sources were aggregated from available datasets across Acadia and synthesized to examine the spatial and temporal patterns of Hg in abiotic (atmosphere, throughfall, litterfall, soils, stream water, and lake sediment) and biotic (birds, mammals, terrestrial and aquatic invertebrates, and fish) matrices of aquatic and terrestrial ecosystems. The synthesis provides evidence that Acadia exhibits decoupled trends in Hg deposition and Hg concentrations in some biotic species. And despite the relatively intensive research conducted to date, there are still significant data gaps and confounding variables that make synthesizing and interpreting Hg patterns in Acadia challenging. These challenges point to the need to develop a robust, reliable, and standardized comprehensive monitoring plan that will allow for synthetic, integrated analyses. The findings from this synthesis have been used to create the outline of a revised research and monitoring plan for Hg in Acadia and other National Parks as they move forward in the face of global change.
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