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Current Conditions of the Northeast U.S. Shelf Ecosystem: Fall 2021 Update
A summary of temperature, salinity, chlorophyll and other conditions on the Northeast U.S. Continental Shelf during the first half of 2020.
A summary of temperature, salinity, chlorophyll and other conditions on the Northeast U.S. Continental Shelf during the first half of 2020. Note that some data elements usually reported in Current Conditions are not available for this report as some of our surveys did not occur as planned. This would include bottom temperature, surface and bottom salinity, and modeling data related to fish distribution and habitat. See previous reports (Spring 2020 and Fall 2019) for these data.
Introduction
“Current Conditions” is a biannual report on ecosystem and fisheries data for the Northeast U.S. continental shelf ecosystem. The report includes measurements of temperature and salinity indicators of seasonal warming and cooling patterns. We derive indicators describing phytoplankton distribution and blooms from satellite remote sensing data. Northeast Fisheries Science Center bottom trawl surveys provide upper trophic level indicators of fish and macroinvertebrate habitat and distribution dynamics. Note that some data elements usually reported in Current Conditions are not available for this report, as some of our surveys did not occur as planned. This would include bottom temperature, surface and bottom salinity, and modeling data related to fish distribution and habitat.
During the first half of 2021, sea surface temperatures in the Northeast U.S. Shelf Ecosystem continued to be above average and was the second highest recorded over the long-term time series. This follows a shift in thermal conditions that began around 2010.
Spring phytoplankton blooms did not form in either the Gulf of Maine or Georges Bank ecoregions in 2021. Chlorophyll concentrations were generally below average throughout the ecosystem.
With continued warming in the ecosystem, the spring thermal transition has trended to earlier date in recent years.
Temperature
Daily Sea Surface Temperature for the First Half of the Year
Daily sea surface temperatures were at or above average during the first half of 2021. The Georges Bank and Gulf of Maine ecoregions had the largest departures from average conditions. Spring temperatures were approximately 2°C above the long-term average. In the Mid-Atlantic, temperatures fluctuated around the mean for the second half of the spring. In all areas except for the Mid-Atlantic Bight, temperatures moderated well above the mean by mid-year.
The 2021 (red) daily sea surface temperature compared with temperatures from 1982–2020 (transparent blue). The dotted lines represent the plus/minus one standard deviation from the 1982–2020 time series mean (black line). Credit: NOAA Fisheries
Sea Surface Temperature Trends for the First Half of the Year
Average sea surface temperatures for the first half of 2021 continued at high levels in all ecoregions. In most areas, thermal conditions were the second warmest since the record high temperatures observed in 2012. Spring temperatures in all areas appear to have experienced a change in level around the year 2010, increasing the mean temperature by 1°C.
Mean sea surface temperature for the first six months of the year. The red lines show the linear trend, the titles indicate the trend’s significance, and the blue lines represent change points. Credit: NOAA Fisheries
The distribution of sea surface temperature by ecoregions was generally above normal. However, monthly sea surface temperature imagery shows that temperatures were below normal in the Mid-Atlantic during the first few months of the year. The strongest temperature anomalies were observed in June in the Gulf of Maine and Georges Bank areas.
Monthly mean sea surface temperature anomalies showing the difference between the monthly 2021 average and the long-term 1982-2020 temperature baseline. Credit: NOAA Fisheries
Trends in Variability of Sea Surface Temperature for the First Half of the Year
Sea surface temperature variability (the standard deviation within each ecoregion) for the first half of 2021 increased in the more northern Northeast Shelf ecoregions. The increase in variability appears to be significant in the Gulf of Maine and Scotian Shelf but not significant in the other areas. There were no clear-cut patterns in any change points in variability between areas.
Variability of sea surface temperature for the first six months of each year. The red lines show the linear trend, the titles indicate the trend’s significance, and the blue lines represent change points. Credit: NOAA Fisheries
Long-term Trends of Sea Surface Temperature for the First Year
We derived the long-term temperature time series from the Extended Reconstructed Sea Surface Temperature dataset. It provides a low-resolution depiction of sea surface temperature on the Northeast Shelf since the 1850s. The data is based on historical shipboard measures and augmented with other data in recent years. The temperature for the last half of 2021 was above average and was the second highest value in the data time series.
The long-term (1854–2021 mean sea surface temperature for the first six months of each year. The red lines show the linear trend, the titles indicate the trend’s significance, and the blue lines represent change points. Credit: NOAA Fisheries
Spring Thermal Transition Day
Phenology is the seasonal timing of plant and animal production cycles. Many marine organisms time their reproductive cycles to make best use of seasonal phytoplankton blooms, such as the spring and fall blooms. In turn, temperature plays a role in the development of blooms. The spring transition date is likely related to different forcing factors by region. The spring thermal transition date was relatively constant from 1982 to approximately 2010 for the Northeast Shelf ecoregions. A change point appeared in the northern areas around 2010, when the transition date advanced approximately two weeks. In recent years, the trend toward earlier spring transition dates appears significant in northern segments of the ecosystem—the Gulf of Maine and Scotian Shelf ecoregions—and the 2021 spring transition in the Gulf of Maine was the earliest in the time series.
The spring temperature transition date. The red lines show the linear trend, the titles indicate the trend’s significance, and the blue lines represent change points. Credit: NOAA Fisheries
Chlorophyll Concentration
Weekly Chlorophyll for the First Half of the Year
Phytoplankton blooms, measured as chlorophyll concentration, are a major component of the food web. They are a primary food source for zooplankton and filter feeders such as shellfish. Chlorophyll concentration was generally below average during the first half of 2020. In areas that typically have a spring bloom, such as Georges Bank and the Gulf of Maine, chlorophyll concentration was below the long-term average through the bloom period. The chlorophyll concentration for the Middle Atlantic Bight and Scotian Shelf also tended to be below the long term mean.
The 8-day mean chlorophyll concentration (2021 in red and 1998–2020 in transparent blue). The dotted lines represent the plus/minus one standard deviation from the 1998–2020 time series mean (black line). Credit: NOAA Fisheries
Chlorophyll Concentration Trends for the First Half of the Year
Average chlorophyll concentrations during the first half of the year appear to have decreased in recent years in all subareas of the ecosystem. With the exception of Georges Bank, the declining trends were significant. It is noteworthy that we have identified a change point in chlorophyll concentration in all areas within the last decade or so.
Mean chlorophyll concentration for the first six months of each year. The red lines show the linear trend, the titles indicate the trend’s significance, and the blue lines represent change points. Credit: NOAA Fisheries
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