7 Mesozooplankton Biomass at Wilkinson Basin
Description: Mesozooplankton biomass at the Wilkinson Basin Time Series Station (WBTS): 2005-2024
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 and Rubao Ji
Affiliations: UMS
7.1 Introduction to Indicator
The Wilkinson Basin Time Series Station (WBTS: 257 m depth), located in the northwest corner of Wilkinson Basin, was established in December, 2004. For about fifteen years it was maintained by PIs at the University of New Hampshire and University of Maine, funded through various short term research projects with several funding gaps in coverage. In 2019, with funding from BOEM and NOPP, the time series was integrated into the Gulf of Maine Marine Biodiversity Observation Network (GoM MBON), administered by NERACOOS. The WBTS station was favored because of the existing time series data, the proximity to coastal ports allowing single-day missions to collect samples and its strategic importance representing the deep western GoM overwintering habitat for the planktonic copepod, Calanus finmarchicus, a key sentinel variable in the GoM pelagic food web. Data collected at the WBTS station include CTD- rosette measurements of salinity, temperature and chlorophyll a concentration, microscopic enumeration of phytoplankton species, bacterial and microplankton measurements using flow cytometer, eDNA measurements, measurement of total mesozooplankton biomass and microscopic enumeration of zooplankton species collected with a 0.75 m, 200µm ring net towed from near bottom to the surface. Only the mesozooplankton biomass data are reported here.
7.2 Key Results and Visualizations
Planktonic copepods typically constituted the great majority of catch of the vertically integrated ring net tow. Notably, at WBTS, the copepodid stages of Calanus finmarchicus typically make up 50% or more of the total mesozooplankton biomass in spring through fall. Larger microzooplankton, like euphausids and jellyfish, are underrepresented. Chaetognaths, round tentaculate ctenophores, notably Pleurobrachia, and salps were captured, although the latter tend to degrade in formaldehyde over time and are likely underrepresented. Given these limitations, the mesozooplankton dry mass data allow comparison of biomass across pelagic ecosystems where similar measurements have been taken.
The annual cycle of mesozooplankton biomass (Fig. 1A) reflects the predominance of C. finmarchicus in the Gulf of Maine mesozooplankton community. At the WBTS station, the lowest biomass (1.52 g/m2: 2012 GAM fit) on March 18th occurs within a day of the nadir in C. finmarchicus abundance. The biomass peak (14.6 g m-2: 2012 GAM fit) occurs on August 6th. High biomass is sustained throughout the late summer by the increasing proportion of the larger and heavier C. finmarchicus stage C5 as well as increased abundance of smaller zooplankton and invertebrate predators (Runge et al. 2023).
Longer term trends in mesozooplankton biomass at the WBTS station mirror the seasonal trends in C. finmarchicus abundance, with significant declines of 64% in fall and 79% in winter between 2005 and 2015 (Fig 1D). By 2023, however, there is evidence for stabilization and a partial rebound in both abundance and biomass, although biomass is still down 65-67% in fall and winter. The estimated mean contribution of C. finmarchicus stages C1-C6 to the total mesozooplankton biomass ranges among years from 40-80% in winter to 50-60% in spring and summer (Runge et al, 2023).
7.3 Indicator statistics
Spatial scale: Wilkinson Basin, representing the western Gulf of Maine
Temporal scale: Annual climatology. Multiannual trends: 2004-2024,spring, summer, fall, winter.
Synthesis Theme:
7.4 Implications
The mesozooplankton biomass follows the seasonal cycle of Calanus finmarchicus abundance. Biomass is at its maximum in the summer months between June and August, averaging 12-15 g/m-2, with the highest biomass observed in July (Fig 1A). The predominant contributor to mesozooplankton biomass is C. finmarchicus stage CV (Runge et al. 2023). In years prior to 2010, when C. finmarchicus CV was abundant in October – December, fall mesozooplankton biomass also attained high levels >25 mg m-2. These biomass levels are as high or higher than mesozooplankton biomass measured across the species’ biogeographic range, including the Gulf of St. Lawrence, the Labrador Sea, Irminger Sea, Iceland Sea and Barents Sea (Casault et al., 2024; Strand et al., 2020; Skjoldal et al., 2022). Runge et al. (2023) report increased abundance of other, smaller omnivorous copepods as well as gelatinous species, which likely is imparting a greater influence on mesozooplankton biomass as C. finmarchicus declines.
A prominent result of the WBTS time series is confirmation of substantial reduction mesozooplankton biomass in fall and winter between 2005-2023, although not in spring or summer, reflecting a fall-winter decline in abundance of the energy-rich stages of the planktonic copepod, Calanus finmarchicus, in the Gulf of Maine.
The primary drivers of C. finmarchicus abundance in the Gulf of Maine, analyzed by Ji et al. (2022), are local production in spring and early summer, external supply from the surface layer Nova Scotia Current and deep water through the Northeast Channel, and predation from both visual (e.g. herring, sandlance) and non-visual (e.g. euphausids, chaetognaths, jellyfish and other invertebrates) predators. The WBTS time series data here indicate a shifting phenology toward higher phytoplankton biomass available in fall, winter and early spring, supporting local production in spring and early summer not only for C. finmarchicus, but also for other, smaller mesozooplankton species (e.g. copepods in the genera Oithona, Pseudocalanus, Centropages and Metridia). This shifting phenology, allowing local late winter-early spring copepod reproduction, is consistent with observations of sustained C. finmarchicus abundance in spring. However, a combination of summer-fall predation and reduced external supply, associated with a shift in transport away from cold, Calanus-rich subarctic water to warm, Calanus-poor Atlantic slope water since 2010 is likely responsible for the substantial reduction in the energy-rich stock of C. finmarchicus in the western Gulf of Maine in summer and fall, the period during which forage fish (e.g. herring and sand lance) as well as North Atlantic right whales, accumulate lipids to sustain their growth and reproduction.
A value of the NERACOOS time series is the timely provision of plankton indicators at seasonal scales. For example, recent data indicate a rebound in Calanus abundance and mesozooplankton biomass, particularly in fall, 2023 and winter 2024 (Fig. 1 D, E), which has implications for the present condition of North Atlantic right whale foraging habitat as well as for forage fish recruitment and condition in the Gulf of Maine. Further monitoring and analysis of hydrographic and other data is needed to understand whether the recent change represents a longer term shift or whether external supply continues to constrain C. finmarchicus abundance in the Gulf of Maine. While time series samples will indicate the abundance trends of some invertebrate predators (e.g. carnivorous copepods and chaetognaths), supplemental data are needed to understand trends in euphausids, jellyfish, and visual predators.
7.5 Get the data
Point of contact: Jeffrey Runge (jeffrey.runge@maine.edu)
ecodata name: No dataset
Variable definitions
Name: mesozooplankton_biomass Definition: g dry weight m^-2
No Data
Indicator Category: