Filtering rates of zebra mussels (Dreissena polymorpha )

and quagga mussels (D. bugensis ) and possible

impacts on 0neida Lake

R. Dunn, M. White and J. Amento

Biology Department, St. Lawrence University, Canton, New York 13617

Introduction

Zebra mussels (Dreissena polymorpha ) and quagga mussels (D. bugensis) are benthic organisms that feed by filtering suspended material, mainly phytoplankton, from the water. Both organisms have an inhalant and exhalent siphon; water is drawn through the inhalant siphon and food material is trapped on a mucous and cilia covered gill. Water and waste material exit through the exhalent siphon.

Both Dreissena polymorpha and D. bugensis are exotic species that were introduced into the Great Lakes system in the mid to late 1980’s. This led to a population explosion which had devastating affects on the phytoplankton levels of the infested bodies of water.

Zebra mussels were the first of the two species to be introduced into the United States and have spread to eight major river systems since their introduction in the Great Lakes. Quagga mussels have been confined to Lake Ontario and the southern shore of Lake Erie. Much is known about the zebra mussels but a lesser degree of information has been published about its close relative the quagga mussel. There is a great fear that the quagga mussels could spread as the zebra mussel has and not knowing their particular biology, it is unclear how quagga mussels may affect aquatic ecosystems (B.S. Baldwin pers. comm.).

A lake that is currently infested with zebra mussels and is being studied is Oneida Lake in central New York. Oneida Lake is a mesotrophic lake that has an area of 207 Km2 and a mean depth of 7 meters. One question raised is what affect quagga mussels would have if introduced into Oneida Lake or a similar body of water.

Our goal was to identify and compare the filtering rates of individual zebra and quagga mussels to hypothesize possible affects on phytoplankton levels. We hypothesize that the quagga mussels will have a higher feeding rate than the zebra mussels and therefore will have more drastic effects on natural habitats such as Oneida Lake.

Methods and Materials

We prepared five liters of filtered Oneida Lake water with an alga, Nannochloris sp. to a concentration of 7.1x105 cells/mL . The concentration was determined using a Coulter Counter. We divided this water into 10 portions of 500 mL and placed each 500 mL portion into a 1000 mL beaker. Each of these five beakers contained 40 zebra mussels and the other five contained 20 quagga mussels. We used more zebra mussels due to the fact that they are significantly smaller than quagga mussels. The size of an individual zebra mussel was roughly 15 mm longitudinally (shell length) while the quaggas were approximately 30 mm longitudinally. Using a higher number of zebra mussels provided us with a more drastic reduction in algal concentrations making it easier to interpret the results over a 60 minute time period. Measurements of algal concentrations were taken at 20 minute intervals, for a period of 60 minutes, starting at time zero (when the organisms were placed in the beakers). The purpose of the experiment was to measure the decline of the algal cell concentrations in the beakers over time due presumably to consumption by the mussels.

An additional liter of filtered Oneida Lake water and algal cells were prepared with the same concentration as above (7.1x105 cells/mL). This liter was separated into two 500 mL samples to use as controls. These beakers contained no organisms. The concentration of these beakers were taken at time zero and at 20 minute intervals similar to the beakers containing the mussels. This was done to determine if settling of the algae was occurring in the control beakers and presumably in the experimental beakers which could have an affect on the perceived concentration of algae in the water.

Filtering rates were determined using the following equation:

FR = Vol [(lnC0 - lnC1) - (lnC’0 -lnC’1)]

tn

where:

FR = filtration rate of mussel per individual (mL/h)

Vol = volume of water in beaker

t = time (h)

n = number of mussels per experimental beaker

C0 = initial algal concentration in experimental beaker (cells/mL)

C1 = final algal concentration in experimental beaker (cells/mL)

C’0 = initial algal concentration in control beaker (cells/mL)

C’1 = final algal concentration in control beaker (cells/mL)

Results

There was a 97% decrease in algal cell concentration over the 60 minute time period for quagga mussels. For this same time period zebra mussel beakers had an average decline of 96% of the algal cells. The control showed a decline in algal concentration of 29% (Figure 1).

Figure 1. Filtering rate patterns of zebra and quagga mussels, compared to the control, measured to determine the rate of settlement of algal cells over a 60 minute time period. Filter rate trends are based on 40 zebra mussels and 20 quagga mussels.

The average filtering rate for zebra mussels was determined to be 37.3 mL/hr/individual while the filtering rate of the quagga mussels was determined to be 81 mL/hr/individual. This was calculated from data based on a 60 minute feeding period. The feeding rates of individual quagga mussels were significantly higher (t-test; p = 4.0 x 10e -8) than those of zebra mussel individuals.

Two scenarios of mussel impact on Oneida Lake were calculated. One, where the mussel population was 100 individuals per square meter giving a total lake population of 2.07 x 10e12 individuals. The second where the mussel population was 10,000 individuals per square meter, 2.07x 10e12 individuals for the entire lake. Based on these population densities, the filtration rates of the populations were determined (Table 1).

TABLE 1. Filtration rates for populations of zebra and quagga mussels. High populations equal 10,000 individuals per square meter. Low populations equal 100 individuals per square meter

Discussion

The quagga mussel had a filtration rate (81 ml/indiv/hr) more than twice that of the zebra mussel (34.3 mL/indiv/h). We infer that this is due to the relative size difference of the two mussels. The overall size of the quagga mussels were roughly twice that of the zebra. The algal cells showed some settling in the control beakers during the experiment. The control beakers helped determine how much of the drop in algae concentration was due to gravity. Although settling lowered the algae concentrations, it was not a significant amount compared with that was filtered by the organisms.

If quagga mussels are able to live successfully in Oneida Lake as the zebra mussels have, they will further the impact on phytoplankton levels initiated by the zebra mussels. The introduction of quagga mussels to this ecosystem will most likely decrease phytoplankton concentrations in the water. Due to the higher feeding rate of the quagga mussel, we believe that the phytoplankton levels would initially drop faster than they did when zebra mussels were first introduced. Based on the filtering rate data, the quaggas seem to have the potential to out-compete zebras for quantity of food (Table 1). If this is the case in a true ecosystem, quagga mussels might have the ability to reduce or even eliminate zebra populations.

We accept our hypothesis that the rate of quagga mussel filtration was higher then that of zebra mussels. Based on the results of this experiment we cannot determine that actual effect of quagga mussels on Oneida Lake because the experiment was run in the laboratory. Although filtering rates are higher for quaggas, we did not take into account the difference in overall size and biomass of the two species. We believe the size and biomass of the organisms could be a more accurate indicator of total filtering rate. Because of the difference in body size, for example 10,000 individual zebra mussels could live within a square meter but maybe only 5,000 quagga mussels could fit into the same area. This would lead us to presume that whether this square meter is inhabited by 10,000 zebras or 5,000 quaggas , the filter rate for organisms within this square meter should be close to each other (based on individual filtering rates).

Over time, with either or both species inhabiting Oneida Lake, phytoplankton levels should drop markedly. This drop in phytoplankton levels will have a adverse affect on the dynamics of Oneida Lake. Because phytoplankton is an important base of the food web, a decrease in numbers will lead to a decrease in populations of suspension feeding organisms that rely on the phytoplankton. In the end this will inevitably mean a decrease in fish populations which are usually the most recognizable sign to the humans.

One possible improvement for this experiment is to measure live tissue weights of each organism and compare filtration rate by tissue mass. Simple comparison by number of organism alone is not enough to draw a conclusion to test the hypothesis. To determine the effects of the mussels' impact on Oneida Lake, it would be very helpful to know how many zebra mussels and how many quagga mussels can inhabit a certain area.