Current Conditions of the Northeast U.S. Shelf Ecosystem: Spring 2024 Update

“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. See archived reports here.

Summary 

• During the last half of 2023, sea surface temperatures in the Northeast U.S. Shelf Ecosystem cooled appreciably, especially along the shelf break, or eastern boundary, of the ecosystem. The only ecoregion remaining at the elevated temperatures typical of recent years was the Gulf of Maine ecoregion.
• Fall phytoplankton blooms appeared to develop in the Gulf of Maine and the Scotian Shelf ecoregions.
• Elevated chlorophyll concentration associated with the dinoflagellate Tripos spp. bloom were also evident, especially in the Gulf of Maine ecoregion.
• The fall thermal transition date shifted to earlier in the year with cooler temperature conditions, reversing the trend of later transitions seen in recent years.

Temperature

Daily Sea Surface Temperature for the Last Half of the Year

Daily sea surface temperatures were generally at average conditions during the last half of 2023. In all ecoregions of the Northeast U.S. Shelf Ecosystem, there were short periods of above average sea surface temperatures; however, with fall cooling all ecoregions fell back to average temperature with the exception of the Middle Atlantic Bight, which fell to below average temperatures.

Sea Surface Temperature Trends for the Last Half of the Year

Sea surface temperatures for the last half of 2023 were lower than the high levels seen in recent years. The decrease in temperature is greatest in Georges Bank, Scotian Shelf, and Middle Atlantic Bight areas, whereas there was little change in temperature in the Gulf of Maine. Despite these changes in temperature in 2023, trends over the period remain unchanged, with all areas suggesting an increasing trend in temperature conditions.

Sea Surface Temperature Distribution for the Last Half of the Year

The distributions of sea surface temperature by ecoregions were mixed with some areas above normal and some areas below. July temperatures were above average throughout the ecosystem. Progressively from September through December, lower temperatures can be seen along the shelf break front area of the ecosystem and in the eastern Gulf of Maine. These changes are likely related to the recent movement of Gulf Stream position.

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 2023 continued increasing trends in sea surface temperature variability in the Gulf of Maine and Middle Atlantic Bight. Temperature variability was without trend in Georges Bank and the Scotian Shelf, with the 2023 value consistent with the lack of trend in those areas.

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 are based on historical shipboard measures and augmented with other data in recent years. The temperature for the last half of 2023 was among the lowest value in over a decade, reflecting some degree of cooling in the ecosystem.

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 all the Northeast Shelf ecoregions. The transition day moderated to earlier in the year in all ecoregions in 2023, reflecting the cooler temperature conditions in fall.

Chlorophyll Concentration

Weekly Chlorophyll for the Last Half of the Year

Chlorophyll concentration was generally above average during the late summer and early fall of 2023. Exceptionally high chlorophyll levels were observed in late summer in the Gulf of Maine, which can be associated with the dinoflagellate Tripos spp. bloom, which dominated the region from April to August. There is evidence of a fall bloom in the Gulf of Maine and Scotian Shelf during the typical fall bloom period.

Chlorophyll Concentration Trends for the Last Half of the Year

Average chlorophyll concentrations during the last half of the year appears to have reached new high levels in 2023 in all areas except the Middle Atlantic Bight. This continues a trend of rebounding chlorophyll concentrations that began 2021. After a period of decline in all ecoregions. The change in chlorophyll concentrations was sufficient to nullify the trends in chlorophyll concentrations established through the 25 year time series.

Fish Distribution from fall Survey Data

The species of the Northeast Shelf ecosystem have shown changes in distribution over recent decades. Individual species have shifted distribution due to a number of reasons and these shifts can be characterized in a number of different ways. Three metrics that have been used to characterize distribution on the NE Shelf include: 1) the position in the ecosystem along an axis oriented from the southwest to the northeast referred to as the along shelf distance; 2) the depth of occurrence; and, 3) the distance from the coastline. Along shelf distances range from 0 to 1360 km, which relates to positions along the axis from the origin in southwest to northeast. Depth ranges from 0 to -260 m, which relates to depth of occurrence. Distance to the coast ranges from 0 to 110 km. These indices are based on 48 of the most abundant taxa. Along shelf distance has increased over time reflecting a shift in species distribution to the Northeast; the 2023 distance was among the highest in the time series. Depth of occurrence has also increased over time reflecting a great use of deeper habitats and was at its deepest level in 2023. There is no significant trend in distance to the coastline. The map position of the center of gravity for these species moved slightly to the northeast in 2023 from its position in 2022.