Implications
-
This is the first of several indicators of trends in Gulf of Maine mesozooplankton abundance and diversity that the NERACOOS MBON (J. Runge/ C. Thompson/ L. Karp-Boss) and NE Shelf LTER (R. Ji) intend to submit to the NOAA SOE process over the next several years. The mean phenology of Calanus finmarchicus based on the NOAA EcoMon/MARMAP data is a starting point, as it provides a baseline from which future change in C. finmarchicus abundance can be interpreted. We have found that it is important to take into account the strong seasonality in abundance associated with the Calanus life cycle (winter spring reproduction, overwintering in the lipid rich late preadult stage in late summer through mid winter) in order to understand abundance trends. The drivers controlling Calanus abundance vary with season and may be synegistic or counteracting in their influence on Calanus abundance, such that an annual index of abundance may mask ecologically important trends. For example, observations from the NERACOOS fixed station in Wilkinson Basin (WBTS) indicate an early timing of food availability in late-winter matching emergence of females from dormancy and resulting in higher spring abundances since 2010. However, advective supply of older copepodid stages into Wilkinson Basin in summer is lower, by as much as 70%, since 2010, reflecting changes in external supply of Calanus into the Gulf of Maine ([14]; [15]) and perhaps also increased predation in the Maine Coastal Current, the proximal source of supply, in summer, associated with higher surface layer temperatures ([16]; [17]).
-
Below are notes submitted to SOE members in November, 2023. The results and discussion are based on a final report submitted by J. Runge and coauthors in fulfillment of an award from BOEM supporting NERACOOS ISMN-MBON plankton observations at two fixed stations, the Coastal Maine Time Series (established 2008, located in mid-coast Maine at the western margin of the Maine Coastal Current) and the Wilkinson Basin Time Series (established 2005, located in the northwest corner of Wilkinson Basin) Stations. We are in the process of analyzing and archiving data for submission to primary journals in 2024, and expect these data to be vetted and available for the SOE process in 2025.
-
NOTES:
-
A NERACOOS-ISMN Marine Biodiversity Observation Network report: Status of the zooplankton in the Gulf of Maine 2023 with focus on Calanus finmarchicus as a sentinel indicator
-
Implications for interpretation of zooplankton data
-
o Copepods are the most abundant taxon in the mesozooplankton captured with 200-333µm mesh nets used in the AZMP, ISMN MBON and EcoMon surveys.
-
o In NW Atlantic, Calanus species dominate the mesozooplankton biomass in waters deeper than 70-100 m ([18]; Johnson et al. NW Atlantic Zooplankton Atlas, in prep.)
-
o The Calanus species found in the Gulf of Maine is Calanus finmarchicus. Apart from Paraeucheata norvegica, a very large carnivorous copepod, C. finmarchicus older stages are considerably larger than other copepods species found in the Gulf of Maine.
-
o C. finmarchicus is supremely adapted to the subarctic North Atlantic. It ingests lipids manufactured by primary producers (especially diatoms) and accumulates them in an oil sac that is used to overwinter in stage CV, the last preadult stage. Most of the zooplankton lipidscape in the Gulf of Maine is in the Calanus late stages, available May-Dec (approx.)
-
o In the decade since the shift in oceanographic conditions that occurred around 2010, C. finmarchicus abundance has declined to 30-40% of its 2005-2008 level in summer-fall in Wilkinson Basin, the center of C. finmarchicus abundance in the Gulf of Maine ([19]). The primary driver of this decline is hypothesized to be a shift in supply of water into the Gulf of Maine starting around 2010, from relatively Calanus rich Scotian Shelf water to relatively Calanus poor Atlantic Temperate Water adjacent to the Gulf Stream. In addition the Calanus abundance on the Scotian Shelf has declined since 2010 [18]. The Calanus seed stock immigrating into the Gulf of Maine is amplified in the Maine Coastal Current, mitigating the reduction in supply, but there is nevertheless a reduction in abundance of the overwintering stock in Wilkinson Basin that is likely also negatively impacting by invertebrate and vertebrate predators.
-
o While summer-winter abundance of C. finmarchicus has declined since 2010, its abundance in spring has not declined ([19]). Abundances were higher in the six years after 2010, before decreasing to pre-2010 levels in the early 2020s. The driver of spring abundance despite a lower overwintering stock is hypothesized to be increased food availability in late winter spring, driving higher reproductive rates generating the spring cohort.
-
o In the period between 2011-2017, after the 2010 oceanographic shift, abundances of many other copepod species have increased (Dullaert et al. in prep), including Centropages typicus, a fall dominant and Pseudocalanus spp., both of which are also prey for North Atlantic right whales in the western Gulf of Maine in spring, before the appearance of abundant lipid-rich late stage Calanus. The likely driver has been the increase in phytoplankton biomass (as measured by Chl. a concentration) in fall and winter and well as increased temperature driving higher population growth rates.
-
o In the period between 2011 and 2017, abundance number of zooplankton taxa other than copepoda also increased ([20]), including Mollusca (pteropods), Ctenophora, Cheatognatha and siphonophores (from EcoMon and ISMN MBON data). The latter three taxa are predators on Calanus and other copepods.
-
o Despite increases in zooplankton other than Calanus, the total mesozooplankton biomass captured in ring net tows has declined, reflecting the predominance of C. finmarchicus. The decline in C. finmarchicus abundance and consequently zooplankton biomass represents lower availability of energy rich lipids to higher trophic levels, since there is no equivalent replacement to C. finmarchicus in the subarctic GoM food web ([19]).
-
o Declines in forage fish (herring condition, sand lance recruitment and NARW foraging) would be expected to follow the declining trend in C. finmarchicus (e.g. [21])
-
o For these reasons, the seasonal indices in Calanus finmarchicus and other zooplankton abundance and biomass are important to track, as are the seasonal drivers: warming trend (surface and bottom waters), sources of external supply, predators and availability of phytoplankton food in late winter. Shifting abundances in zooplankton taxa have been observed in previous decades in the EcoMon survey (e.g. [22]; [23]). There has been no radical regime shift (e.g. a speculation that lipid-rich menhaden replace Calanus, with consequences for the structure of the higher trophic levels), but given the trend in increasing CO2 and ocean temperatures, close observation of change in the lower trophic levels in the GoM is warranted.
+
This is the first of several indicators of trends in Gulf of Maine mesozooplankton abundance and diversity that the NERACOOS MBON (J. Runge/ C. Thompson/ L. Karp-Boss) and NE Shelf LTER (R. Ji) intend to submit to the NOAA SOE process over the next several years. The mean phenology of Calanus finmarchicus based on the NOAA EcoMon/MARMAP data is a starting point, as it provides a baseline from which future change in C. finmarchicus abundance can be interpreted. We have found that it is important to take into account the strong seasonality in abundance associated with the Calanus life cycle (winter spring reproduction, overwintering in the lipid rich late preadult stage in late summer through mid winter) in order to understand abundance trends. The drivers controlling Calanus abundance vary with season and may be synegistic or counteracting in their influence on Calanus abundance, such that an annual index of abundance may mask ecologically important trends. For example, observations from the NERACOOS fixed station in Wilkinson Basin (WBTS) indicate an early timing of food availability in late-winter matching emergence of females from dormancy and resulting in higher spring abundances since 2010. However, advective supply of older copepodid stages into Wilkinson Basin in summer is lower, by as much as 70%, since 2010, reflecting changes in external supply of Calanus into the Gulf of Maine ([14]; [15]) and perhaps also increased predation in the Maine Coastal Current, the proximal source of supply, in summer, associated with higher surface layer temperatures ([16]; [17]).
+Below are notes submitted to SOE members in November, 2023. The results and discussion are based on a final report submitted by J. Runge and coauthors in fulfillment of an award from BOEM supporting NERACOOS ISMN-MBON plankton observations at two fixed stations, the Coastal Maine Time Series (established 2008, located in mid-coast Maine at the western margin of the Maine Coastal Current) and the Wilkinson Basin Time Series (established 2005, located in the northwest corner of Wilkinson Basin) Stations. We are in the process of analyzing and archiving data for submission to primary journals in 2024, and expect these data to be vetted and available for the SOE process in 2025.
+
NOTES:
+A NERACOOS-ISMN Marine Biodiversity Observation Network report: Status of the zooplankton in the Gulf of Maine 2023 with focus on Calanus finmarchicus as a sentinel indicator
+
Implications for interpretation of zooplankton data
+o Copepods are the most abundant taxon in the mesozooplankton captured with 200-333µm mesh nets used in the AZMP, ISMN MBON and EcoMon surveys.
+o In NW Atlantic, Calanus species dominate the mesozooplankton biomass in waters deeper than 70-100 m ([18]; Johnson et al. NW Atlantic Zooplankton Atlas, in prep.)
+o The Calanus species found in the Gulf of Maine is Calanus finmarchicus. Apart from Paraeucheata norvegica, a very large carnivorous copepod, C. finmarchicus older stages are considerably larger than other copepods species found in the Gulf of Maine.
+o C. finmarchicus is supremely adapted to the subarctic North Atlantic. It ingests lipids manufactured by primary producers (especially diatoms) and accumulates them in an oil sac that is used to overwinter in stage CV, the last preadult stage. Most of the zooplankton lipidscape in the Gulf of Maine is in the Calanus late stages, available May-Dec (approx.)
+o In the decade since the shift in oceanographic conditions that occurred around 2010, C. finmarchicus abundance has declined to 30-40% of its 2005-2008 level in summer-fall in Wilkinson Basin, the center of C. finmarchicus abundance in the Gulf of Maine ([19]). The primary driver of this decline is hypothesized to be a shift in supply of water into the Gulf of Maine starting around 2010, from relatively Calanus rich Scotian Shelf water to relatively Calanus poor Atlantic Temperate Water adjacent to the Gulf Stream. In addition the Calanus abundance on the Scotian Shelf has declined since 2010 [18]. The Calanus seed stock immigrating into the Gulf of Maine is amplified in the Maine Coastal Current, mitigating the reduction in supply, but there is nevertheless a reduction in abundance of the overwintering stock in Wilkinson Basin that is likely also negatively impacting by invertebrate and vertebrate predators.
+o While summer-winter abundance of C. finmarchicus has declined since 2010, its abundance in spring has not declined ([19]). Abundances were higher in the six years after 2010, before decreasing to pre-2010 levels in the early 2020s. The driver of spring abundance despite a lower overwintering stock is hypothesized to be increased food availability in late winter spring, driving higher reproductive rates generating the spring cohort.
+o In the period between 2011-2017, after the 2010 oceanographic shift, abundances of many other copepod species have increased (Dullaert et al. in prep), including Centropages typicus, a fall dominant and Pseudocalanus spp., both of which are also prey for North Atlantic right whales in the western Gulf of Maine in spring, before the appearance of abundant lipid-rich late stage Calanus. The likely driver has been the increase in phytoplankton biomass (as measured by Chl. a concentration) in fall and winter and well as increased temperature driving higher population growth rates.
+o In the period between 2011 and 2017, abundance number of zooplankton taxa other than copepoda also increased ([20]), including Mollusca (pteropods), Ctenophora, Cheatognatha and siphonophores (from EcoMon and ISMN MBON data). The latter three taxa are predators on Calanus and other copepods.
+o Despite increases in zooplankton other than Calanus, the total mesozooplankton biomass captured in ring net tows has declined, reflecting the predominance of C. finmarchicus. The decline in C. finmarchicus abundance and consequently zooplankton biomass represents lower availability of energy rich lipids to higher trophic levels, since there is no equivalent replacement to C. finmarchicus in the subarctic GoM food web ([19]).
+o Declines in forage fish (herring condition, sand lance recruitment and NARW foraging) would be expected to follow the declining trend in C. finmarchicus (e.g. [21])
+o For these reasons, the seasonal indices in Calanus finmarchicus and other zooplankton abundance and biomass are important to track, as are the seasonal drivers: warming trend (surface and bottom waters), sources of external supply, predators and availability of phytoplankton food in late winter. Shifting abundances in zooplankton taxa have been observed in previous decades in the EcoMon survey (e.g. [22]; [23]). There has been no radical regime shift (e.g. a speculation that lipid-rich menhaden replace Calanus, with consequences for the structure of the higher trophic levels), but given the trend in increasing CO2 and ocean temperatures, close observation of change in the lower trophic levels in the GoM is warranted.