Current Conditions of the Northeast U.S. Shelf Ecosystem: Fall 2024 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. This fall update reports on conditions in the first half of 2024. See archived reports here.

Summary

• Long-term trend of warming sea surface temperatures across the region persists;
• However, during the winter of 2024 (January-March), we observed anomalously cold surface temperatures in the northern ecoregions: Georges Bank, Gulf of Maine, and Scotian Shelf. This seems to have been driven mostly by colder-than-usual water along shelf break;
• In spring 2024, sea surface temperatures rose rapidly, such that by May and June temperatures were above historical averages in all ecoregions. Surface temperatures in the Gulf of Maine throughout May and June were particularly high.
• Strong phytoplankton blooms were observed in all ecoregions in mid to late April, with an especially intense bloom in the Gulf of Maine. In general, mean chlorophyll concentrations appear to be recovering from all-time lows in 2019.

Temperature

The following suite of indicators analyzes temperature trends using data from the Optimum Interpolation Sea Surface Temperature (OISST) records. 

Daily Sea Surface Temperature for the First Half of the Year

In 2024, we observed a step change in sea surface temperatures relative to historical values: temperatures were colder than usual in the first few months of the year, and then became warmer than average by May. We do see some spatial differences: colder waters were observed in the northern portion of the region – Georges Bank, Gulf of Maine, and the Scotian Shelf – but temperatures in the Middle Atlantic Bight remained near historical values. High spring temperatures were most pronounced in the Gulf of Maine, with values more than one standard deviation above average. The Scotian Shelf and Middle Atlantic Bight also saw warm temperatures during May and June, while Georges Bank had closer-to-average values.

Sea Surface Temperature Trends for the First Half of the Year

Long-term warming trends continue across all four ecoregions. Winter-spring (i.e., Jan-June) temperatures in all areas appear to have experienced a step change in level around 2010, increasing the mean temperature by 1°C. In each region, Jan-June 2024 was the coolest of any of the last 5 years.

Sea Surface Temperature Distribution for the First Six Months of 2024

Spatial patterns within each ecoregion help explain the general sea surface temperature trends described above. In January through May, the shelf break regions of the Middle Atlantic Bight and Georges Bank are anomalously cold. Inshore waters tended to be warmer than average. In June, this spatial gradient broke down and warm waters were observed across the shelf.

The Gulf of Maine and Scotian Shelf tended to be more spatially homogeneous, with cool surface waters across these regions in January and February transitioning to anomalously warm in March and persisting through June. June surface temperatures were particularly warm in the Gulf of Maine: approximately 3°C higher than the historical average.

Trends in Variability of Sea Surface Temperature for the First Six Months of 2024

Variability in sea surface temperature in both the Gulf of Maine and Scotian Shelf has shown a linear increase since the 1980s. No trends are observed on Georges Bank or the Middle Atlantic Bight. For the Gulf of Maine and the Scotian Shelf, variability in January-June was the highest in 2024 than any previous year analyzed.

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. With this longer-term view, we observe a significant increase in sea surface temperatures on the Northeast Shelf.

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 and fall blooms. In turn, temperature plays a role in the development of blooms. Identifying the initiation date of the spring transition temperature, which varies by region, can help determine the physical forcing on spring transition events. The spring thermal transition date here is defined as the first day when the 5-day running average of sea surface temperature was above the long-term (1982-2011) average for each region. This date was relatively constant from 1982 to approximately 2008 for the Northeast Shelf ecoregions. A change point appears around 2008, when the spring transition date shifts earlier by approximately one to two weeks. The linear trend to earlier spring transition dates is significant in all the Northeast Shelf ecoregions except the Middle Atlantic Bight, which also shows the least dramatic 2008 regime shift. In comparison to last year, the 2024 spring transition day moderated to earlier days of the year in the Gulf of Maine and Scotian Shelf, was about the same in Georges Bank, and was later in the Middle Atlantic Bight.

Chlorophyll Concentration

Weekly Chlorophyll for the First Six Months of 2024

Chlorophyll concentration was generally at or above average during the first six months of 2024, in all ecoregions. An exceptionally high spring bloom was observed in late April in the Gulf of Maine. There is evidence of a slightly late spring bloom in Georges Bank and the Scotian Shelf.

Chlorophyll Concentration Trends for the First Six Months of 2024

Average chlorophyll concentrations during the first half of the year appears to have reached new high levels in 2024 in the Middle Atlantic Bight and remained well above average in the Gulf of Maine and Scotian Shelf, down only slightly from last year’s record highs. This continues a trend of rebounding chlorophyll concentrations following a period of decline in all ecoregions. These recent increases in chlorophyll concentrations were sufficient to nullify the downward trends in chlorophyll concentrations established through the initial two decades of the time series.

Fish Distribution from Spring Bottom Trawl Survey Data

The species of the Northeast Shelf ecosystem have shown changes in distribution over recent decades. Individual species have shifted for various reasons and these shifts can be characterized in a number of different ways. We characterize fish distributions using: 1) the position along the coast travelling southwest to northeast, which we refer to as “along shelf distance”; and, 2) the depth where the fish are found. These indices are based on 48 of the most abundant taxa in the bottom trawl surveys, and here we present results from analyses of spring surveys. Along shelf distance has increased over time reflecting a shift in species distribution toward the northeast. Depth of occurrence has also increased over time reflecting a greater use of deeper habitats.