Bacteria

HarpacticoidGymnamoeba

Trophic Relationships among Bacteria, Gymnamoebae

(Protozoans) and Harpacticoids in the Sediments of Otsego Lake

Paul J. Bischoff – Secondary Science Education

Thomas G. Horvath-Biology

Harpacticoids

Harpacticoids are benthic (bottom-dwelling) copepods (Crustacea) measuring usually < 1 mm. This group of animals have been poorly studied in freshwater ecosystems. However, their ecological role in marine and estuary ecosystems suggests that they are key players in linking the benthic with the pelagic (open water) food webs. Harpacticoids are most likely bactivorous, but also may be feeding on the abundant protozoans present in the benthic sediments. In Otsego Lake’s nearshore areas, we believe harpacticoids to be univoltine (one complete life cycle per year), and they will encyst for unknown periods. Preliminary evidence suggests they encyst during summer and reproduce during winter.

The nearshore zone of lakes is an area that is not well understood. Pelagic, or open-water, studies have far outnumbered studies on the shallow lake bottoms, most likely because the pelagic systems are homogenous and the benthic systems very heterogenous. Despite the lack of studies, nearshore shallow zones of lakes are extremely important systems in terms of understanding how lakes function ecologically. For example, nearly all fish spend some part of their life in nearshore areas, particularly the young-of-the-year fish. These fish feed primarily from benthic invertebrates and eventually venture out into the open waters. Thus, the nearshore community is a vital link between the energy resources in the benthic zone and the energy sources in the pelagic zone. Understanding the components of this link is necessary to our understanding of lake ecosystems.

The community in the nearshore benthic zone of a typical lake is comprised mostly of macroinvertebrates (>500 µm), meiofauna (45-500µm), single-celled protozoans, and microbes (bacteria and fungi). All limnology textbooks indicate the importance of the "microbial loop" to the pelagic system. In this loop, bacteria are using the dissolved organic matter excreted by the higher organisms, the bacteria are eaten by protozoans, which are eaten by zooplankton, which are eaten by the higher organisms. Some evidence from stream ecosystems suggests that a similar trophic pathway exists in the benthic sediments. Very few data are available on the protozoan-meiofauna relationship.

Specific objectives:1) Evaluate numerically and categorically the abundances of gymnamoebae inhabiting sediments in Otsego Lake.2) Estimate the densities of bacteria in the sediments of Otsego Lake and examine relationships in bacterial and gymnamoebae densities. 3) Determine if a trophic (energy) relationship exists between and among harpacticoids, bacteria and gymnamoebae inhabiting the sediments of Otsego Lake. 4) Determine the feeding preferences for a single species of harpacticoid common to Otsego Lake.

TREATMENT

Experimental Design

CONTROL

• Sediments collected from Otsego Lake• 20 ml of unfiltered lake water in each microcosm

– Meiofauna removed via elutriation– Still contains bacteria & protozoans

• Each test ran for 96 hours

• N=10, June through August• Differences tested with t-test.