Life in the Water Column

From NOAA’s NMFS Ecology of the Northeast Continental Shelf Toward an Ecosystem Approach to Fisheries Management, courtesy of Michael Fogarty.

Spatial distribution patterns of zooplankton numbers over two time periods 1977-1987 and 1997-2002
Spatial distribution patterns of zooplankton numbers over two time periods 1977-1987 and 1997-2002*

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While many species live most or all of their lives on or near the seafloor, a rich community of animals spends their lives in the water column itself. This is called the pelagic part of the ecosystem. Planktonic species, schooling pelagic fishes, marine mammals, sea turtles and top predators inhabit an environment primarily defined by current systems, frontal zones, and other oceanographic structures. These ever-changing features of the physical geography of the sea are every bit as important to the ecosystem as a whole as are seabed habitats.

Zooplankton

As with the phytoplankton, we see distinct geographical patterns in the distribution of zooplankton species. These patterns mirror the distribution of primary production to a significant degree. The highest zooplankton counts are found in the nearshore regions of the mid-Atlantic Bight and on the central crest of Georges Bank. The lowest counts occur in the Gulf of Maine and on the shelf edge in general. We have observed changes over time in the structure of the diverse communities inhabiting the water column. For example, a generally increasing trend in overall zooplankton abundance has been observed on the Northeast Shelf since the mid-1980s.

Changes in the relative abundance of different zooplankton species over time have also been observed, with certain groups favoring warmer water temperatures now dominating the system. In areas such as the North Sea, it has been suggested that changes in the composition of plankton communities are linked to recruitment success of species such as cod, since larval cod prey on zooplankton.

Why it Matters – A Bridge Between Large and Small

Apex predators such as bluefin tuna are key members of the pelagic ecosystem
Apex predators such as bluefin tuna are key members of the pelagic ecosystem***

Understanding events in the water column is key to determining changes in the survival of young fish – recruitment – and also important transfer of energy between different parts of the system. Zooplankton are a bridge to larger animals in the system from fish to whales. Factors affecting their abundance can make the difference between good and poor survival for these groups. Again, oceanographic features figure prominently in defining the pelagic ecosystem, emphasizing the importance of understanding the system in its entirety from physics to biology and on to the human dimension.

Representation of tidal mixing front on Georges Bank
Representation of tidal mixing front on Georges Bank. Frontal zones are areas of high concentration of plankton and fish**

Food and Survival

In recent years, we have also seen shifts in the timing of when key zooplankton species populations start to peak during the year. The small crustacean Calanus finmarchicus is one of the most important species in the planktonic copepod community throughout the North Atlantic. The period of peak abundance of this species in the Gulf of Maine and on the Scotian Shelf has shifted to earlier in the spring, and lasts longer than has been typical in the past.

Living at the Front

As we have noted, many species forage in oceanographic structures such as frontal zones where their prey are concentrated. For example large shoals of herring are often found at tidal mixing fronts where high densities of their zooplankton prey are found. In turn, fishing activities are often concentrated in these areas to capitalize on these natural associations between predators and their prey for commercially important species.

*Zooplankton map by Joseph Kane NMFS, NEFSC
**Illustration by Gregory Lough NMFS, NEFSC, and Paul Oberlander WHOI;
***Tuna photo by Greg Skomal, MADMF