Current Conditions of the Northeast U.S. Shelf Ecosystem: Fall 2019 Update

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.

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Summary

• During the first half of 2019, sea surface temperatures in the Northeast U.S. Shelf Ecosystem continued to be moderate compared to the record high temperatures that occurred in 2012. However, based on contemporary satellite remote-sensing data and shipboard measurements, temperatures remain above the long-term average.
• The Gulf of Maine and Georges Bank 2019 winter-spring phytoplankton blooms were  below average.
• Compared to recent years, the spring thermal transition dates for 2019 were near average. 
• According to bottom trawl survey samples through spring 2019, fish and invertebrate species distribution has shifted to the northeast and into deeper water. 

Temperature

Daily Sea Surface Temperature for the First Half of the Year

Daily sea surface temperatures were at or below average conditions during the first half of 2019. The Scotian Shelf ecoregion had the largest departures from average conditions. Early in the year, temperatures were approximately 2°C below the average in the Scotian Shelf area before returning to average temperatures by mid-year. In contrast, temperatures in the Middle Atlantic Bight, Gulf of Maine, and Georges Bank stayed close to average level through most of the first half of the year.

The 2019 (red) daily sea surface temperature compared to temperatures from 1982–2018 (transparent blue).  The dotted lines represent the plus/minus one standard deviation from the 1982–2019 time series mean (black line).

Sea Surface Temperature Trends for the First Half of the Year

Average sea surface temperatures for the first half of 2019 were generally at average levels in all ecoregions. However, thermal conditions have become more moderate since the record high temperatures observed in 2012. Though the other ecoregions’ linear trends indicate increasing temperatures, none of them appear to be significant. However, all areas show an abrupt change in temperature in the last decade, which may moderate with the 2019 data. 

Annual 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.

Trends in Variability of Sea Surface Temperature for the First Half of the Year

Sea surface temperature variability (the standard deviation) for the first half of 2019 increased in all 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.

Annual 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. 

Long-term Trends of Sea Surface Temperature for the First Half Year

The long-term temperature time series is derived 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 temperatures for the first half of 2019 were above average and generally match conditions during the warm period of the late 1940s and early 1950s. 

The long term (1850–2019) annual 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.

Bottom Temperature Trends for the First Half Year 

The thermal conditions at the bottom of the water column are extremely important in defining the habitats for the majority of resource species. While sea surface temperatures can be measured by satellite remote sensors, bottom temperatures must be measured from ship surveys and other means. For this reason we often have incomplete spatial and temporal sample coverage to describe bottom water temperatures. The bottom temperatures for the first half of the year have been increasing in all ecoregions, but this trend appears to be significant only in the Scotian Shelf. There were no suggestions of any change points in temperature in the Middle Atlantic Bight. However, there were change points in the Scotian Shelf and Georges Bank areas around 2010. 

The annual mean bottom 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.

Spring Thermal Transition Day

Phenology is the climate’s influence on the timing between plant and animal production cycles. Many marine organisms time their reproductive cycles to make best use of seasonal phytoplankton blooms, like the spring bloom. In turn, temperature plays a role in the development of the spring bloom. Identifying the initiation date of the spring transition temperature, which varies by region, can help determine the average temperature between winter and summer. 

The spring thermal transition date was relatively constant from 1982 to approximately 2010 for the Northeast Shelf ecoregions. A change point appears in all areas around 2010, when the spring transition date advanced approximately two weeks. The trend shift to earlier spring transition dates appears significant in the Gulf of Maine and Scotian Shelf regions.

The annual 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.

Salinity

Salinity and temperature can be indicators of the source water in the ecosystem. 

Surface Salinity Trends for the First Half of the Year

Surface salinity, measured in practical salinity units, has increased in all ecoregions except Georges Bank. However, in some areas there appears to have been a regime shift in 2012, when salinities increased by approximately 0.5 practical salinity units. In all areas there is the suggestion that surface salinity decreased to below average levels in 2019.

The annual mean surface salinity for the first half of the year.  The red lines show the linear trend, the titles indicate the trend’s significance, and the blue lines represent change points.

Bottom Salinity Trends for the First Half of the Year

Bottom salinity has increased in all ecoregions except the Gulf of Maine. In most regions there appears to have been a regime shift in 2012, as salinities increase by approximately 0.5 practical salinity units.

The annual average bottom salinity for the first half of the year. The red lines show the linear trend, the titles indicate the trend’s significance, and the blue lines represent change points.

Chlorophyll Concentration

Weekly Chlorophyll for the First Half of the Year

Chlorophyll concentration was below average during the first half of 2019. In areas that typically have a spring bloom, such as Georges Bank, chlorophyll concentration was below the long-term average through the bloom period. The chlorophyll concentration for the Gulf of Maine spring bloom was above average for one week, but well below average for most of the bloom period.

The 2019 (red) 8-day mean chlorophyll concentration compared to the concentrations from 1998–2018 (transparent blue).  The dotted lines represent the plus/minus one standard deviation from the 1998–2019 time series mean (black line).

Chlorophyll Concentration Trends for the First Half of the Year

Average chlorophyll concentrations during the winter-spring months appear 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 and the Gulf of Maine. It is noteworthy that we have identified a change point in chlorophyll concentration in all areas within the last decade or so.

Annual average 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.

Trends in Species Distribution Metrics

Distribution of Northeast U.S. Shelf species has changed over recent decades. Individual species have shifted distribution for a number of reasons. We can characterize these shifts with the following metrics including the center of gravity of: 

1. Depth 
2. Distance to the coastline
3. Latitude
4. Longitude
5. Along-shelf position in the ecosystem along an axis oriented from the southwest to the northeast

The averages of these metrics for 48 of the more abundant Northeast Shelf species are shown in time series plots below. All these metrics have increased significantly with the exception of distance to the coast, indicating a continuing shift of species distribution to the northeast and into deeper water.

Annual time series of species distribution metrics (a) depth, (b) distance to the coastline, (c) latitude, (d) longitude, and (e) along shelf distance. The linear trend is shown in red, with an indication of the significance of the trend listed in the title, and change point indicators are shown in blue.

Trends in Fall Occupancy Habitat by Functional Group

Habitat or species distribution models provide estimates of the area a particular species would be likely to occupy based on their preferred habitat. We analyzed the output of a series of these models to determine the trends in the area of 50% probability of occurrence habitat by functional group. To ameliorate the effect of more abundant and widely distributed species, we processed the habitat area time series 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). The trend for pelagic piscivores was not significant.

Annual time series plots of the total area where the probability of occurrence is greater than 50 percent. The linear trend is shown in red and the change point indicator in blue.

Ecosystem Forecast-Experimental Data Product

As weather and earth system models have improved, so have monthly forecasts over seasonal scales ranging up to seven months in advance. For each of the ecoregions of the Northeast Shelf, we provide forecasts from an ensemble of seven forecast models starting with estimates for November 2019 and ending in May 2020.

The forecasts suggest that the Northern ecoregions will have above average sea surface temperatures, while the Middle Atlantic Bight will progressively experience less elevated temperatures. 

Learn more about these ecosystem forecasts

Mean monthly forecasts (red line) of sea surface temperature compared to the all of the forecasts in the ensemble (transparent blue lines).  The plus/minus one standard error from the mean is shown in the dotted line.

Other Reports

• Spring 2019 Current Conditions Report
• Spring 2020 Current Conditions Report
• 2006 to 2018 Archived Current Conditions Reports