Current Conditions of the Northeast U.S. Shelf Ecosystem: Spring 2021 Update

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.

Summary

During the last half of 2020, sea surface temperatures in the Northeast U.S. Shelf Ecosystem continued to be above average following a shift in thermal conditions that began around 2010.
Fall phytoplankton blooms were generally below average and it is questionable whether a fall bloom developed in any part of the ecosystem
The fall thermal transition continues to shift to progressively later on the year, raising concern over the timing of production cycles in the food web.

Temperature

Daily Sea Surface Temperature for the Last Half of the Year

Daily sea surface temperatures were at or generally above average conditions during the last half of 2020. The Georges Bank, Gulf of Maine, and Scotian Shelf ecoregions had the largest departures from average conditions where mid-summer temperatures were approximately 3°C above the average. In the Middle Atlantic Bight areas temperatures fluctuated around one standard deviation above the mean. In all areas, temperatures tended to moderate toward the mean by the end of the year.

Graphs showing the daily sea surface temperatures for all years between 1982 and 2020 for the last six months of the year. Top Row: Middle Atlantic Bight and Georges Bank. Bottom Row: Gulf of Maine and Scotian Shelf. — The 2020 (red) daily sea surface temperature compared to temperatures from 1982–2019 (transparent blue). The dotted lines represent the plus/minus one standard deviation from the 1982–2020 time series mean (black line).

Sea Surface Temperature Trends for the Last Half of the Year

Sea surface temperatures for the last half of 2020 continued at high levels in all ecoregions. However, thermal conditions have become more moderate since the record high temperatures observed in 2012. The linear trends in all the ecoregions appear to be significant.

Graphs showing sea surface temperature for the last six months of the year from 1998 to 2020. Top Row: Middle Atlantic Bight and Georges Bank. Bottom Row: Gulf of Maine and Scotian Shelf — Mean sea surface temperature for the last 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.

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

Sea surface temperature variability (the standard deviation) for the last half of 2020 increased in the coastal boundary ecoregions. The increase in variability appears to be significant in the Gulf of Maine but not significant in the other areas. There were no clear-cut patterns in any change points in variability between areas.

Graphs showing standard deviation of sea surface temperature for the last six months of the year from 1998 to 2020. Top Row: Middle Atlantic Bight and Georges Bank. Bottom Row: Gulf of Maine and Scotian Shelf. — Variability of sea surface temperature for the last 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 Last 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 temperature for the last half of 2020 was above average and generally matched the conditions during the warm period of the late 1940s and early 1950s.

Graph showing the mean sea surface temperature for the last six months of the year from 1850 to 2020 for the Northeast Shelf. Range 14 to above 18 degrees Celsius. — The long-term (1850–2020) mean sea surface temperature for the last 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.

Fall 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 and fall blooms. In turn, temperature plays a role in the development of blooms. Identifying the initiation date of the fall transition temperature, which varies by region, can help determine the physical forcing on fall transition events. The fall thermal transition date was relatively constant from 1982 to approximately 2008 for the Northeast Shelf ecoregions. A change point appears in the northern areas around 2008, when the fall transition date advanced approximately two weeks. The trend to earlier fall transition dates appears significant in the Gulf of Maine, Georges Bank, and Scotian Shelf regions.

Graphs showing fall temperature transition date from 1998 to 2020. Top Row: Middle Atlantic Bight and Georges Bank. Bottom Row: Gulf of Maine and Scotian Shelf. — The fall temperature transition date. 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 Last Half of the Year

Chlorophyll concentration was generally below average during the last half of 2020. In areas that typically have a fall bloom, such as Georges Bank, chlorophyll concentration varied below the long-term average through the bloom period. The chlorophyll concentration for the Gulf of Maine appear above average late in the bloom period suggested a bloom might have occurred.

Graphs showing the 8-day mean chlorophyll concentration from 1998 to 2020 for the last six months of the year. Top Row: Middle Atlantic Bight and Georges Bank. Bottom Row: Gulf of Maine and Scotian Shelf. — The 8-day mean chlorophyll concentration (2020 in red and 1998–2019 in transparent blue). The dotted lines represent the plus/minus one standard deviation from the 1998–2020 time series mean (black line).

Chlorophyll Concentration Trends for the Last Half of the Year

Average chlorophyll concentrations during the last half of the year appears to have decreased in recent years in all subareas of the ecosystem. The most dramatic decline appears to have occurred in the Middle Atlantic Bight and Georges Bank areas. It is noteworthy that we have identified a change point in chlorophyll concentration in all areas within the last decade or so.

Graphs showing mean chlorophyll concentration for the last six months of the year from 1998 to 2020. Top Row: Middle Atlantic Bight and Georges Bank. Bottom Row: Gulf of Maine and Scotian Shelf. — Mean chlorophyll concentration for the last 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.

Last updated by Northeast Fisheries Science Center on 04/11/2022