Zooplankton of Great Bay Estuary

Zooplankton

Plants and animals that float passively or swim weakly in water are known as plankton. A wide variety of marine organisms share this way of life.

Animal and animal-like free-floaters are known as zooplankton. They come in a range of sizes: from single-cell protozoa, to minute crustaceans, to basketball-sized jellyfish. Unable to produce food, many feed on phytoplankton, those planktonic organisms that produce food through photosynthesis.

Some species are temporary zooplankton, also known as meroplankton. For example, many crustaceans, mollusks, worms, and fish are planktonic only as they grow through their larval phases.

The species that are permanent plankton are called holoplankton. A study in the mid-1970's (Normandeau Associates, Inc.) found that the holoplankton make up more than 70 percent of the estuary's plankton population. Of these, copepods in their first larval stage were the most common, then adults of a few other copepod species. These tiny (usually less than an eight of an inch) crustaceans have barrel-shaped bodies and big antennae, which they use to strain diatoms out of the water. They form an important base of the estuary’s food web. Microscopic tintinnids and the rotifers, which use hair-like appendages called cilia to move and eat, have also been found.

Thirty-two species of zooplankton have been found in Great Bay Estuary. They can be found at predictable times and in predictable places throughout the estuary.

The greatest diversity and density of meroplankton larvae occur from February to July. The northern rock barnacle spawns first, and its population peaks in mid-to-late March. This coincides with a spring phytoplankton bloom, prompted by the warmth, the sunlight and the nutrient-rich waters occuring after winter dormancy. The larvae of various species float and feed for up to a month each, until water temperatures rise in late summer. When the water cools in September, another, smaller spawning of meroplanton feeds on the fall phytoplankton bloom.

How do these larvae end up finding the right place to settling down as they are carried in tidal currents that could disperse them for miles in different directions? Remember, conditions vary widely at different ends of the estuary. At the mouth the Piscataqua the water conditions are cooler and more saline, just right for coastal fauna like sea stars, blue mussels and northern rock barnacles. But at the upper end of the estuary, in Great Bay, temperatures and salinities are extremely variable—conditions tolerated by far fewer species.

Donna DeMoranville Turgeon, a UNH doctoral student in the mid-1970's, found while researching her thesis that there are two communities of larvae at opposite ends of the estuary. Even though these larvae swim weakly, they migrate to surface waters at certain stages of the tide to maintain proximity to adult populations of their kind. It is thought that changing currents and salinity levels may prompt vertical migrations in estuarine meroplankton.

The highest densities of meroplankton of the lower estuary are found in the upper water column during high tides. There are also higher concentrations of phytoplankton and less turbid water there. Flood tide carries the larvae upstream, preventing them from being flushed out of the estuary. When the tide ebbs, the larvae drop down in the water column for transport back to the vicinity where they started. These vertical migrations, combined with the oscillations of the tide back and forth across the estuary, apparently keep these larvae in the lower parts of the estuary where they might find a suitable habitat.

At the other end of the estuary, another meroplankton community rises to surface waters in the greatest numbers at the slack period after low tide. At this point, they can feed on higher densities of fresh and brackish plankton, and avoid strong currents and increased water turbidities. Plus, the vertical migration prevents the estuarine larvae from being drawn by ebb tides beyond the midpoint of the estuary. The numbers of these larvae decline as you move downstream.

In that study, greater numbers of holoplankton were also found on the flooding tides in mid-estuary.