6 Seasonal Variation of Calanus finmarchicus
Description: The data presented here are abundance estimates (no./m2) of the planktonic copepod, Calanus finmarchicus, collected at the NERACOOS-MBON Wilkinson Basin Time Series (WBTS) station between 2005-2023
Indicator family:
Contributor(s): Jeffrey A. Runge, Cameron R.S. Thompson, Shawn Shellito, Emma C. Dullaert, Isabel A. Honda, Douglas Vandemark, Dylan Pugh, Riley Young-Morse, Jackie Motyka, Rebecca J. Jones, Lee Karp Boss, Rubao Ji
Affiliations: UMS
6.1 Introduction to Indicator
Mesozooplankton (predominantly copepods) graze on phytoplankton and smaller zooplankton and serve as the foundation of production for higher trophic levels. In deeper waters of the Gulf of Maine (>100m), the net-captured mesozooplankton biomass is dominated by the planktonic copepod, Calanus finmarchicus, a signature species of the subarctic ecosystem spanning the North Atlantic between the Gulf of Maine in the south and west to the Norwegian and Barents Seas in the north and east. Supremely adapted to the seasonality of the subarctic North Atlantic, the lipid-rich older stages of mesozooplankton nourish and influence condition of forage fish such as Atlantic herring, sand lance and Atlantic mackerel, as well as serve as the primary prey for the endangered North Atlantic right whale, whose fecundity is dependent on sufficient availability of C. finmarchicus in late spring through fall.
While the EcoMON/MARMAP surveys provide valuable observation and insight into the changes in abundance of C. finmarchicus in the Gulf of Maine over time, the regional observing community has recognized the need for more frequent water column sampling at the subannual scale for better documentation and understanding of change in phenology and seasonal abundance of C. finmarchicus, especially given seasonally varying primary drivers (Ji et al., 2010; Runge et al., 2012; NERACOOS, 2016; Ji et al., 2022). In 2005, monthly sampling for zooplankton and associated environmental variables was initiated at a station in the northwest corner of Wilkinson Basin, presently known as the Wilkinson Basin Time Series (WBTS) Station. This fixed station, representing the primary deep overwintering habitat for C. finmarchicus in the western Gulf of Maine, is accessible by day trips with a coastal research vessel from Portsmouth, NH. Sampling at the WBTS station was constrained in the first two decades by the availability of funding from short-term research grants, resulting in some gaps in coverage. More recently, the stations are on more secure footing when the Northeastern Regional Association of Coastal Ocean Observing Systems (NERACOOS), a part of the U.S. IOOS program, assumed administrative oversight and successfully entered the stations in the U.S. Marine Biodiversity Observation Network (MBON) in 2020. Data on a number of environmental and plankton biodiversity variables are now collected as part of the routine sampling at the two stations. (Runge et al., 2023).
The indicator of C.finmarchicus abundance here has two parts. An annual climatology of abundance of older stage (C3-C6) abundance shows how the abundance of the copepod varies seasonally in Wilkinson Basin over the course of a year. The second part of the indicator shows trends by season in C. finmarchicus C3-C6 abundance in Wilkinson Basin between 2005, the start of the time series and 2023. It is important to show trends by season because the primary drivers determining abundance of the species are seasonally variable, such that abundance increased over time or stayed the same (spring) but decreased in fall and winter.
Only the late copepodid stages (C3-adult) of C. finmarchicus are shown in this indicator, but these stages nevertheless are representative of the seasonal variation in abundance of the population and the data are also comparable to data from the MARMAP/EcoMon surveys, which use a larger mesh size than the ring nets employed in the NERACOOS-MBON time series. This indicator serves as a baseline that can be used to interpret future changes in western Gulf of Maine C. finmarchicus abundance.
6.2 Key Results and Visualizations
The annual climatology graph (Fig. 1A) shows the change in abundance of C.finmarchicus over a mean annual cycle in Wilkinson Basin, representing the western Gulf of Maine. The annual life cycle of C. finmarchicus is consistent with previous studies of the C. finmarchicus life cycle across in the North Atlantic, characterized by a pronounced seasonal variation in the composition of its developmental stages. Abundance is highest in spring and early summer, when young stages are prevalent, and lowest in March, when the population is comprised of mostly adults surviving diapause. The difference between the nadir and peak in abundance (6,000 m-2 on March 17 and 86,000 m-2 on May 31: GAM fit for 2012) is an order of magnitude.
There was no trend in abundance of stage C3-C6 C. finmarchicus in Wilkinson Basin in spring between 2005-2023. (Fig. 1B), although our analysis indicates that inclusion of the young stages C1-C2 would show a significant increase in spring production in the years immediately after 2010. In summer, stage C3-C6 abundances were highest, up to 250,000 m-2 in the period 2010-2017, but fell to lower levels in 2020-2021 and overall no change was detected (Fig. 1C). However, significant and dramatic declines of C. finmarchicus copepodid abundance occurred in fall and winter (Fig.1D, E). Between 2005 and 2015 abundance declined 55% and 80% in the fall and winter respectively. Fall and winter abundance continued to decline through 2020 and remains low in 2023, but the trend shows signs of a recent rebound. The counterbalancing effect of no trend in larger numbers of C. finmarchicus in spring and summer and declines in spring and fall results in the absence of an overall trend in a GAM analysis of C. finmarchicus abundance over the time series on an annual basis.
6.3 Indicator statistics
Spatial scale: Wilkinson Basin, representing the western Gulf of Maine
Temporal scale: Spring, summer, fall, winter: 2005-2023
Synthesis Theme:
6.4 Implications
Historically, Wilkinson Basin in the western Gulf of Maine has harbored one of the most abundant shelf populations of C. finmarchicus across its biogeographic range, particularly during the late summer-winter overwintering period (Melle et al., 2014). The high numbers of C. finmarchicus in the western Gulf of Maine represent the fortuitous consequence of several geographic features and processes, including a circulation that provides supply from Calanus rich waters to the north, optimal temperature and food availability in summer in the Maine Coastal Current and a basin well situated to receive local production, deep enough to support overwintering but too shallow for mesopelagic predators residing in the adjacent North Atlantic Ocean. The primary drivers of the high abundance are seasonal in their prominence (Ji et al.,2022): food availability for local production, external advective supply from inflowing waters in the Nova Scotia Current and/or deep water through the Northeast Channel, and predation from both visual (e.g. herring, sand lance) and non-visual (e.g., carnivorous copepods, euphausids, chaetognaths, jellyfish and hyperiid amphipods) predators.
The 18-year time series at the WBTS station spanned a major shift in oceanographic conditions occurring around 2010 (Record et al., 2019; Meyer Gutbrod et al., 2021; Townsend et al., 2023). Trends in C. finmarchicus stage structure and abundance and environmental drivers show the response of this species to this shift. In spring of the years following 2010, early copepodid stages appeared sooner and were more abundant. In fall and winter, late-stage C. finmarchicus abundance declined by 60-80% between 2005 and 2023 (Fig. 1). Overall, these results broadly corroborate previously published analyses of NOAA EcoMon and CPR data in the eastern GoM (Record et al., 2019; Meyer Gutbrod et al., 2021; Ji et al.,, 2022; Shank et al., 2024) and western Gulf of Maine (Pershing and Kemberling, 2024; Honda et al., 2024).
Food availability as a driver is particularly important in the annual cycle of C. finmarchicus during winter and spring in Wilkinson Basin. The timing and magnitude of production of the new generation in late winter/spring is determined by the match or mismatch between exit from diapause of the overwintering stock and availability of food for egg production (Maps et al., 2012; Durbin et al., 2003). Extensive empirical studies indicate that C. finmarchicus egg production rates are very sensitive to chlorophyll a concentration (presented as mean chlorophyll a liter-1 or as chlorophyll a standing stock in the upper 50 m, which when divided by 50 m yields the mean concentration) found in winter and early spring. Egg production rate is near zero at concentrations < 0.5 µg/liter, then increases fourfold between 0.5 and 1 µg/liter (Runge et al., 2006). Mean winter chlorophyll a concentrations were above the 0.5 µg/liter threshold between 2011 and 2024 and below the threshold between 2004-2010. These seemingly small changes in chlorophyll concentration have potentially large impacts on C. finmarchicus population dynamics. At the WBTS station, there was the phenological shift between 2010-2015 toward earlier and higher local, late winter/early spring reproduction and growth of nauplius and copepodid stages. While spring abundances at the WBTS station were highly variable across years, reflecting in part the timing of collection dates with respect to the order of magnitude increase in C3-C6 abundance during the spring period, the observations indicate increased (2012-2016) or at least sustained spring C. finmarchicus production, such that there is no significant change in stage C3-C6 abundance across the time series.
The primary drivers of C. finmarchicus abundance in Wilkinson Basin in fall and winter are likely advective upstream supply (Runge et al., 2015; Ji et al., 2017; Ji et al., 2022) and losses due to predation (Ji et al., 2022; Wiebe et al., 2022; Pershing and Kemberling, 2024). A declining supply of C. finmarchicus could be driven by lower seed stock in the eastern Gulf of Maine, modulated to some extent by subsequent amplifying reproduction and growth in the Maine Coastal Current (Ji et al., 2017. The Western Scotian Shelf is likely the primary source of C. finmarchicus into the Gulf of Maine. The flow of Scotian Shelf Water (SSW), colder, fresher water in the Nova Scotia Current, predominates in spring to late summer into the eastern Gulf of Maine (Townsend et al., 2023). Scotian Shelf water is Calanus-rich (Casault et al., 2024) in spring and flows into the Gulf of Maine during the season when C. finmarchicus are actively growing. Observations by the Canadian AZMP program indicate that C. finmarchicus abundance has been markedly lower since 2010, notably by one to two orders of magnitude in 2011-12 along the Browns Bank Line, where the Nova Scotia current enters the eastern Gulf of Maine, but also during other years since 2010 (Casault et al., 2024: Fig. 21). Lower C. finmarchicus in the Nova Scotia Current is likely an important source of decline not only to C. finmarchicus abundance in the eastern Gulf of Maine, but also to C. finmarchicus transported to Wilkinson Basin in the Maine Coastal Current in summer and fall. While our analysis of trends of invertebrate predators at the WBTS station is still ongoing, this result provides evidence for the hypothesis of increasing invertebrate predation contributing to declining C. finmarchicus abundance in fall and especially prominent in winter when supply from the Maine Coastal Current is greatly diminished.
During all seasons between 2005-2023 there were significant multiannual trends in water temperature at the WBTS station (Fig. 8). In spring, water temperature increased at depth and less so at the surface (Fig 6A). Summer temperatures fluctuated over the time series, ending somewhat higher. Fall temperatures fluctuated in the deep layer and slightly increased elsewhere in the water column. Winter temperatures increased at depth, but there was no significant change in the surface layer. In all cases, the temperature variation over the 18 year time series duration was less than 1-2°C. A temperature increase from 5-7°C, say, raises C. finmarchicus egg production and growth rates and chaetognath consumption rates by about 20% (Runge and Plourde, 1996; Davis, 1984). The most important conclusion from these observations is therefore that temperature by itself had a relatively minor impact on Calanus physiological rates and overall ecological impact. At this point in time temperature changes remain in the optimum range for C. finmarchicus population growth in the Gulf of Maine, although diapause duration may be affected (Maps et al., 2012).
6.5 Get the data
Point of contact: Jeffrey Runge (jeffrey.runge@maine.edu)
ecodata name: No dataset
Variable definitions
Name: Calanus finmarchicus abundance_WBTS station
Definition: number of Calanus finmarchicus stages C3-C6 m^-2
No Data
Indicator Category: