Current Conditions of the Northeast Shelf Ecosystem: Spring 2019 Update
A summary of temperature, salinity, chlorophyll and other conditions on the Northeast U.S. Continental Shelf during the second half of 2018
Introduction
The Current Conditions is a biannual report on ecosystem and fisheries indicators for the Northeast U.S. continental shelf ecosystem. The report includes measurements of temperature and salinity indicators of seasonal warming and cooling patterns. Indicators describing phytoplankton distribution and blooms are derived from satellite remote sensing data. Upper trophic level indicators of fish and macroinvertebrate habitat and distribution dynamics are based on the Northeast Fisheries Science Center bottom trawl surveys.
Summary of Conditions of the Northeast Shelf Ecosystem
- During the second half of 2018, sea surface temperatures in the Northeast Shelf Ecosystem were well above average in the summer but had moderated to average levels by the end of the year.
- Bottom temperature has increased steadily in all ecoregions of the Northeast Shelf, resulting in what appears to have been a regime shift in benthic thermal conditions in 2008.
- The fall bloom in the Gulf of Maine and Georges Bank areas were of average dimension in 2018.
- The fall thermal transition continues to occur later in the year in the more northern segments of the ecosystem. The area of fall occupancy habitat has expanded for the vast majority of species occurring on the Northeast Shelf, suggesting increasing temperature may be transforming habitats that were once outside the thermal tolerance of species.
Daily Sea Surface Temperature
Second Half of the Year
Daily sea surface temperatures tended to be at or above average conditions during the second half of 2018. The largest departures from average conditions can be seen in the data for the Scotian Shelf and Gulf of Maine ecoregions (see graphs). In these plots, the current year temperatures are shown as a red line with all previous years in the dataset, starting in 1982, shown as transparent blue lines; the black line is the time series mean and the dotted lines mark plus/minus one standard deviation. From mid-year into fall, temperatures in these areas were approximately 3°C above the mean before returning to level of at or below the mean by the end of the year. In contrast, the elevated summer temperatures in the Middle Atlantic Bight and Georges Bank tended to be approximately one standard deviation above the mean before moderating to average levels.
Sea Surface Temperature
Second Half of the Year
The OISTT sea surface temperature data is based on satellite measurements and provides high spatial and temporal resolution depictions of temperature trends. The SST conditions for the second half of 2018 were generally at above average levels in all ecoregions, however, noting that thermal conditions have moderated since the record high temperatures recorded in 2012. The Gulf of Maine, Georges Bank, and Middle Atlantic Bight SST time series appears to have two change point features, suggesting abrupt changes in temperature around 1999 and 2011 (see graph). The Scotian Shelf area has only one change point that occurred in 2008. These time series plots contain the linear trend shown in red with an indication of the significance of the trend in the title and a change point indicator shown in blue. The linear trends in all the ecoregions indicate increasing temperatures; all the trends were significant.
Variability in Sea Surface Temperature
Second Half of the Year
The OISTT sea surface temperature data is based on satellite measurements and provides high spatial and temporal resolution depictions of temperature trends. The variability in sea surface temperature conditions for the second half of year only appears to be increasing in the Gulf of Maine ecoregion (see graph); trends in all other areas were non-significant. These time series plots contain the linear trend shown in red with an indication of the significance of the trend in the title and a change point indicator shown in blue. The increasing trend in the Gulf of Maine is also ecoregion is reflected in potential change points in 1987 and 2006; there were no meaningful change points in the other ecoregions.
Long-term Sea Surface Temperature
Second Half of the Year
The ERSST temperature time series provides a low-resolution depiction of sea surface temperature on the Northeast Shelf since the 1850s and is based on historical shipboard measurements, augmented with other data in recent years. The SST for the second half of 2018 was above average and generally matched the sea surface temperatures seen during the warm period of the late 1940s and early 1950s (see graph). These time series plots contain the linear trend shown in red with an indication of the significance of the trend in the title and a change point indicator shown in blue.
Bottom Temperature
Second Half of the Year
The thermal conditions at the bottom of the water column are extremely important in defining the habitats for the majority of resource species. Unlike sea surface temperatures that can be measured synoptically with satellite telemetry, bottom temperatures must be measured directly from ship surveys and other means. Thus, we often have incomplete spatial and temporal sample coverage to describe bottom temperature conditions. Recently, scientists at the NEFSC developed an interpolation approach that provides a more accurate depiction of bottom temperatures. The temperature conditions for the second half of year has been increasing significantly in all ecoregions (see graphs). These time series plots contain the linear trend shown in red with an indication of the significance of the trend in the title and a change point indicator shown in blue. There were change points detected in all regions around the year 2008.
Fall Thermal Transition Day of the Year
Phenology is the climate influence on the timing between plant and animal production cycles. Many marine organisms time their reproductive cycles or migrations to best utilize seasonal production cycles. One measure to characterize the change in the timing of thermal forcing is the date of arrival of a fall transition temperature, which will vary by region and is meant to mark the average temperature between summer and winter. The date of arrival of the fall thermal transition temperature has been relatively constant since 1982 to approximately 2009 for the Northeast Shelf ecoregions (see graphs). These time series plots contain the linear trend shown in red with an indication of the significance of the trend in the title and a change point indicator shown in blue. The trend in transition day appears significant in all ecoregion with the exception of the Middle Atlantic Bight.
Bottom Salinity
Second Half of the Year
Salinity and temperature provide valuable indicators of the nature of source water present in the ecosystem. Salinity, in the units of practical salinity units or PSU, has increased in all ecoregions except on Georges Bank (see graphs). These time series plots contain the linear trend shown in red with an indication of the significance of the trend in the title and a change point indicator shown in blue. However, in all areas there appears to have been a regime shift in salinity in 2012 as salinities increase by approximately 0.5 PSU. In all areas there is the suggestion that salinity may have returned to lower levels in 2018.
Weekly (8-day time step) Chlorophyll Concentration
Second Half of the Year
Chlorophyll concentration tended to be at or below average conditions during the second half of 2018. In areas that typically have a fall bloom, such as the Gulf of Maine and Georges Bank, chlorophyll concentration was at the long-term mean through the bloom period (see graph). In these plots, the current year chlorophyll concentrations are shown as a red line with all previous years in the dataset, starting in 1998, shown as transparent blue lines; the black line is the time series mean and the dotted lines mark plus/minus one standard deviation. In the Middle Atlantic Bight, chlorophyll concentration was generally at average levels; whereas, the Scotian Shelf area was well below average for most of the bloom period.
Chlorophyll Concentration
Second Half of the Year
Chlorophyll concentration appears to have decreased in recent years in all areas of the ecosystem. The most dramatic decline appears to have occurred in the Middle Atlantic Bight where the trend is significant (see graph). These time series plots contain the linear trend shown in red with an indication of the significance of the trend in the title and a change point indicator shown in blue. It is noteworthy that a change point in chlorophyll concentration has been identified in all areas around the year 2012.
Trends in Fall Occupancy Habitat
by Functional Group
Habitat or species distribution models provide estimates of the extent of an ecosystem a particular species would likely occupy. The output of a series of these models was analyzed to determine the trends in the area of 50% probability of occurrence habitat by functional group. To ameliorate the effect of the more abundance and widely distributed species, the habitat area time series were processed with principal component analysis to provide a single index that reflects the change among all members of the functional group. The trend among benthivores, demersal piscivores, and planktovores were all significant and positive (see graphs). These time series plots contain the linear trend shown in red with an indication of the significance of the trend in the title and a change point indicator shown in blue. The trend for pelagic piscivores was non-significant.
Ecosystem Forecast
Experimental Data Product
As weather and earth system models have improved, monthly forecasts over seasonal scales ranging up to seven months in advance have improved in quality. For each of the ecoregions of the Northeast Shelf, forecasts from an ensemble of seven forecast models are provided starting with the forecast estimates for June 2019 and ending in December 2019. The forecasts suggest the Northern subareas will have above average thermal conditions whereas the Middle Atlantic Bight will progressively experience less elevated temperatures (see graphs). In these plots, the mean forecasts are shown as a red line with all model forecasts in the ensemble shown as transparent blue lines; the dotted lines mark plus/minus one standard error from the mean. For more information about these forecasts, visit the North American Multi-Model Ensemble home website.
