This review provides an integrated synthesis with timelines and evaluations of ecological responses to eutrophication in Chesapeake Bay, the largest estuary in the USA. Analyses of dated sediment cores reveal initial evidence of organic enrichment in ~200 yr old strata, while signs of increased phytoplankton and decreased water clarity first appeared ~100 yr ago. Severe, recurring deep-water hypoxia and loss of diverse submersed vascular plants were first evident in the 1950s and 1960s, respectively. The degradation of these benthic habitats has contributed to declines in benthic macroinfauna in deep mesohaline regions of the Bay and blue crabs in shallow polyhaline areas. In contrast, copepods, which are heavily consumed in pelagic food chains, are relatively unaffected by nutrient-induced changes in phytoplankton. Intense mortality associated with fisheries and disease have caused a dramatic decline in eastern oyster stocks and associated Bay water filtration, which may have exacerbated eutrophication effects on phytoplankton and water clarity. Extensive tidal marshes, which have served as effective nutrient buffers along the Bay margins, are now being lost with rising sea level. Although the Bay's overall fisheries production has probably not been affected by eutrophication, decreases in the relative contribution of demersal fish and in the efficiency with which primary production is transferred to harvest suggest fundamental shifts in trophic and habitat structures. Bay ecosystem responses to changes in nutrient loading are complicated by non-linear feedback mechanisms, including particle trapping and binding by benthic plants that increase water clarity, and by oxygen effects on benthic nutrient recycling efficiency. Observations in Bay tributaries undergoing recent reductions in nutrient input indicate relatively rapid recovery of some ecosystem functions but lags in the response of others. KEY WORDS: Eutrophication · Nutrients · Chesapeake Bay Resale or republication not permitted without written consent of the publisherChesapeake Bay is a large estuary which has undergone many changes in its ecological properties and processes in response to nutrient enrichment over the last 2 centuries. Susceptibility of the Bay to eutrophication arises in part from the long dendritic shoreline that intimately connects it to its large watershed (covering an area 15 times that of the Bay) which contains expanding human population centers and extensive agricultural activities. (Satellite image from MODIS,

dominated by strong seasonality in sea ice and water temperatures. Extremely warm conditions from 2017 into 2019 -including loss of ice cover across portions of the region in all three winters -were a marked change even from other recent warm years. Biological indicators suggest th is state change could alter ecosystem structure and function. Here we report observations of ke y physical drivers, biological responses, and consequences for humans, including subsisten ce hunting, commercial fishing, and industrial shipping. We consider whether observed state changes are indicative of future norms, whether an ecosystem transformation is alread y underway, and if so, whether shifts are synchronously functional and system-wide, or reveal a slower cascade of changes from the physical environment through the food web to huma n society. Understanding of this observed process of ecosystem reorganization may shed light on transformations occurring elsewhere.The highly productive northern Bering and Chukchi marine shelf ecosystem has long been

Some of the longest and most comprehensive marine ecosystem monitoring programs were established in the Gulf of Alaska following the environmental disaster of the Exxon Valdez oil spill over 30 years ago. These monitoring programs have been successful in assessing recovery from oil spill impacts, and their continuation decades later has now provided an unparalleled assessment of ecosystem responses to another newly emerging global threat, marine heatwaves. The 2014–2016 northeast Pacific marine heatwave (PMH) in the Gulf of Alaska was the longest lasting heatwave globally over the past decade, with some cooling, but also continued warm conditions through 2019. Our analysis of 187 time series from primary production to commercial fisheries and nearshore intertidal to offshore oceanic domains demonstrate abrupt changes across trophic levels, with many responses persisting up to at least 5 years after the onset of the heatwave. Furthermore, our suite of metrics showed novel community-level groupings relative to at least a decade prior to the heatwave. Given anticipated increases in marine heatwaves under current climate projections, it remains uncertain when or if the Gulf of Alaska ecosystem will return to a pre-PMH state.

Analyses of a 16 year time-series (1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000) of mesozooplankton abundance in the Chesapeake Bay reveal the influence of freshwater flow on species composition and abundance. Trend analysis and linear mixed-effects regression models were used to assess long-term variation in, and influence of water-quality parameters (modulated by freshwater input) on, the monthly mean abundance of the 2 dominant copepod species Acartia tonsa and Eurytemora affinis. There were no long-term trends in abundance of either copepod species, with the exception of a slight downward trend for A. tonsa in the mesohaline region of the northern Chesapeake Bay. There were no longterm trends in temperature, but there was a downward trend in salinity related to a wet period in the late 1990s. Linear mixed-effects models showed a negative correlation between freshwater input and A. tonsa abundance in the oligohaline region, and no significant relationship between other waterquality parameters and A. tonsa abundance in the mesohaline region. A. tonsa abundance was positively correlated with temperature in the polyhaline region. E. affinis abundance in the oligohaline region was negatively correlated with biovolume of the ctenophore Mnemiopsis leidyi and positively correlated with phytoplankton abundance. A negative correlation with salinity and a positive correlation with turbidity were found for E. affinis in the mesohaline region. While years of above-average freshwater input were characterized in the spring by high abundances of E. affinis in the mesohaline region and low abundances of A. tonsa in the oligohaline region, the former may show a lagged response depending on the time and magnitude of the input. Freshwater input appears to be mainly influencing habitat parameters specific to each copepod species and top-down control by predators.