Current Conditions of the Northeast U.S. Shelf Ecosystem: Fall 2023 Update

The report includes selected indicators of the physical and biological environment focusing on changes in either the first or second half year and reflective of data collections made in seasonal scientific surveys. This fall update reports on conditions in the first half of 2023. You may use this link to access archived reports.

Summary

• During the first half of 2023, sea surface temperatures in the Northeast U.S. Shelf Ecosystem continued to be well above average. A shift in thermal conditions began around 2010 with a step change of nearly 1°C higher temperatures in most subareas of the ecosystem.
• The spring thermal transition continues a trend of earlier initiation of spring conditions in the Northeast Shelf, also displaying a step change in transition dates around the year 2010.
• Spring 2023 phytoplankton blooms in the Gulf of Maine and Scotian Shelf ecoregions were exceptional; the blooms were associated with very high levels of chlorophyll concentration and they persisted past the typical spring bloom time frame into the summer season.
• The increased chlorophyll concentrations in these ecoregions appear to be caused by a widespread bloom of the dinoflagellate Tripos spp., which is considered unusual for the Gulf of Maine.
• Fish and macroinvertebrate distributions could not be described in spring 2023 due to an incomplete survey caused by vessel and staffing issues. The absence of these data compromises fishery assessments and our ability to understand climate change effects in the region.
• An El Niño event that appears to be underway in the Pacific Ocean will be monitored for any effects it may have on the Northeast Shelf Ecosystem.

Temperature

Daily Sea Surface Temperature for the First Half of the Year

Daily sea surface temperatures were at or above average conditions during the first half of 2023. The largest departures from average conditions were seen in the Georges Bank and Scotian Shelf ecoregions during late winter. Temperatures were two standard deviations above the mean for extended periods. Temperature moderated to mean levels by late spring to early summer, more so in the Middle Atlantic Bight, where most of June was at the mean temperature level for that ecoregion.

Sea Surface Temperature Trends for the First Half of the Year

Average sea surface temperatures for the first half of 2023 continued at high levels in all ecoregions. In Georges Bank, Gulf of Maine, and Scotian Shelf, the thermal conditions were nearly as warm as the record high temperatures observed in 2012. Spring temperatures in all areas appear to have experienced a change in level around 2010, increasing the mean temperature by 1°C.

Sea Surface Temperature Distribution for the First Half of the Year

The distributions of sea surface temperature by ecoregions were generally above normal. However, monthly sea surface temperature imagery shows that temperatures were below normal in the Mid-Atlantic during June. The strongest temperature anomalies were observed in Georges Bank area along its southern flank.

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

Sea surface temperature variability (the standard deviation within each ecoregion) for the first half of 2023 increased in the more northern Northeast Shelf ecoregions. The increase in variability appears to be significant in the Gulf of Maine but non-significant in the other areas. There were no clear-cut patterns in any change points in variability between areas.

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

The sea surface temperature for the first half of 2023, based on long-term shipboard observations and other data sources, was above average and among the highest values in the data time series beginning in 1854. As also seen using remote sensing data sources, 2012 was the warmest temperature in the time series. Change points in the time series delineate a warm period in the 1940s into the 1950s that, until recent temperatures changes, had been the warmest period in the thermal history of the ecosystem.

Spring Thermal Transition Day

Phenology is the seasonal timing of plant and animal production cycles. Many marine organisms time their reproductive cycles to make best use of seasonal phytoplankton blooms, such as the spring and fall blooms. Temperature can play a role in the development of these seasonal blooms. Identifying the initiation date of the spring transition temperature, which varies by region, is likely related to different forcing factors. The spring thermal transition date was relatively constant from 1982 to approximately 2010 for all the Northeast Shelf ecoregions. A change point appeared in the northern areas around 2010, when the transition date advanced approximately two weeks. In recent years, the trend toward earlier spring transition dates appears significant in northern segments of the ecosystem—the Gulf of Maine, Scotian Shelf, and Georges Bank ecoregions.

Chlorophyll Concentration

Weekly Chlorophyll for the First Half of the Year

Phytoplankton blooms, measured as chlorophyll concentration, are a major component of the food web. They are a primary food source for zooplankton and filter feeders such as shellfish. Chlorophyll concentration was generally above average during the first half of 2023. There was what could be described as an intense and persistent spring bloom in the Gulf of Maine that began in March and continued through June. This bloom pattern was highly atypical.

Chlorophyll Concentration Trends for the First Half of the Year

Average chlorophyll concentrations during the first half of the year decreased in recent years in all subareas of the ecosystem to a low point in 2019. However, this pattern has been reversed in recent years with the 2023 levels at the highest observed in the Gulf of Maine and Scotian Shelf. These record levels are about twice the concentration typically observed over the time series. Chlorophyll concentration increased dramatically on Georges Bank and in the Middle Atlantic Bight, but the increase only approximates levels seen earlier in the time series. The increased chlorophyll concentrations in the Gulf of Maine in the latter half of the spring can be attributed to a widespread dinoflagellate bloom of Tripos spp. The genus of the bloom species was first identified in April, but it can be assumed to have been present earlier. Several institutions in the Gulf of Maine region are investigating the cause of the bloom and the potential consequences of the event on the ecosystem.

Possible Effects of El Niño on the Northeast Shelf Ecosystem

There is concern that an El Niño event is developing in the Pacific Ocean this year and is predicted to continue into the winter. These events have effects that extend beyond the Pacific basin via what are termed “teleconnections”. The 1997 to 1998 event, shown in the Oceanic Niño Index (ONI) time series and marked with a dashed vertical line in the figure at left below, may have had teleconnections to the Northeast Shelf Ecosystem. The plot of Mid-Atlantic Bight days for quantile four, in the figure at right below, is a count of warm temperature days and shows a change point in this index coincident with the 1997 to 1998 El Niño event, also marked with a dashed vertical line. This transition period in physical conditions for the Northeast Shelf aligns with shifts in zooplankton community structure and fish recruitment.