Calanus finmarchichus, a foundation species for the ecosystem

Supporting the Gulf of Maine community at the top of the food web is a planktonic community with its own subarctic characteristics.  We have identified a planktonic copepod, Calanus finmarchicus, as a key foundation species in the Gulf of Maine food web (Figure 2). One of the region’s largest copepod species, C. finmarchicusis remarkably abundant throughout its subarctic North Atlantic range and especially in the Gulf of Maine, where it serves a primary prey for herring and other forage fish and for the endangered North Atlantic right whale, which can feed on millions of the rice-sized life stages of the copepod each day.

 

The life history of Calanus finmarchicusis perfectly adapted to the subarctic seasonal cycle of primary production. The copepods start reproduction in late winter and early spring as the planktonic algal production is increasing. As they develop, Calanusaccumulates lipids produced by the planktonic algae, and by its last preadult life stage it has stored a large quantity, relative to its overall body size, of the liquid high energy fats that enable it to overwinter at depth starting in late summer.  Herring, right whales and other planktivores in the Gulf of Maine are tuned to the C. finmarchicuslife cycle and eat large quantities of the lipid rich stages to fuel their energy needs and supply materials for production of offspring.  In a sense, C. finmarchicus serves as a battery in the Gulf of Maine coastal ecosystem, storing in the water column the light energy converted by planktonic algae that in turn supports the higher Gulf of Maine community.

 

Zooplankton sampling

The sampling is focused on a representative site, Wilkinson Basin Time Series (WBTS: 42.86 N, 69.86 W; depth: 225 m; see Figure 1).

In mid August 2016, a joint UMaine-WHOI team, consisting of 2 scientists, a technician and a postdoc conducted zooplankton sampling onboard the University of New Hampshire's R/V Gulf Challenger. Using a 0.25 square meters Multiple Opening and Closing Net, with an Environmental Sensing System (MOCNESS) and a Video Plankton Recorder (VPR), we measured the vertical distribution of C. finmarchicus at a station in the Maine Coastal Current and in Wilkinson Basin.

 

Modeling

The individual-based model (IBM) is offline coupled to a high-resolution regional ocean model, Finite Volume Community Ocean Model-Gulf of Maine Version-3 (FVCOM-GOM3), and consists of two components: (1) a Lagrangian tracking model; (2) a copepod life cycle model. The Lagrangian tracking model solves the advection equation using a fourth order Runge-Kutta (RK-4) method. The stage- based life cycle model resolves copepod life stage progression through all 13 developmental stages based on temperature- and food-dependent development time (or rate). Satellite ocean color chlorophyll-a data are used for retrieving food conditions.

Results

Model results

Observations

 

Coastal Amplification of Supply and Transport (CAST)

Combining the models and the observational evidence, a conceptual model of Coastal Amplification of Supply and Transport (CAST) dynamics is illustrated here (Figure 8). The Bay of Fundy (BoF) is a potential source region that supplies the seed population to the downstream Maine coast and eventually reaching Wilkinson Basin (WB).  The C5 individuals reach WB during early-mid summer (Fig 8, left panel) could be the direct offspring of the BoF adults or through one additional generation; whereas the C5 individuals reaching WB in later-summer and fall are more likely to go through 1 or 2 more generations.  It is worth noting that this conceptual model needs to be investigated in future studies, ideally through a direct high-resolution observation of population structure along the Maine Coastal Current (MCC), such that the along-shelf stage succession from egg to nauplii and further to copepodites can be detected.