Table of Contents
- Section 1
- Section 2
- Section 3
- Section 4
- Section 5
- Section 6
- Section 7
- List of Figures
- List of Tables
The Effect of Zebra Mussels on Cycling and Potential
Bioavailability of PCBs: Case Study of Saginaw Bay
The modeling effort under this contract is an extension of the previously carried out work by Limno-Tech., Inc. (1995, 1997). Earlier Bierman and Dolan (1981, 1986a, 1986b) have developed a eutrophication model to address issues related to nutrients and nuisance algal blooms. This water quality model was used to establish the target phosphorus loading to Saginaw Bay as part of the 1978 Great Lakes Water Quality Agreement. Recently, this model was expanded to include zebra mussels (Limno-Tech., Inc. 1995, 1997) under the coupled modeling framework of multi-class phytoplankton-zebra mussel model for Saginaw Bay (SAGZM). It was used to investigate water quality responses of the bay to changes in external phosphorus loadings and zebra mussel densities (Limno-Tech., Inc. 1995, 1997; Bierman et al. 1998).
The present report contains the modeling work on the effect of zebra mussels on cycling and potential bioavailability of polychlorinated biphenyls (PCBs) in Saginaw Bay. The motivation of this study was to synthesize the understanding about the ecology and bioenergetics of zebra mussels so as to aid management decisions and to direct future research goals on their ecosystem level impacts.
Historically, mathematical modeling of aquatic resources within the Great Lakes has focussed on assessment and evaluation of management strategies for individual management issues. But the actions directed towards one management area can impact other problems. For effective management, eutrophication and contamination problems have to be combined and an understanding of the interactions between trophic state and contaminant concentration is essential. To understand and quantify the effects of zebra mussels on ecosystems, it would be valuable to develop a modeling synthesis that includes both eutrophication and fate and transport of toxic chemicals. The present study was aimed to develop a Multi-stressor Aquatic Ecosystem Model, which incorporates the ecosystem linkages by coupling models of heretofore separate problem domains.
The approach taken was to develop a modeling framework (SAGZM/PCB) by coupling particle dynamics, PCB dynamics, and bioaccumulation models to SAGZM to account for inter-linked ecological problems. A major shortcoming of SAGZM is that it does not represent the whole spectrum of particles that are important for PCB dynamics in an ecosystem rather a portion of it. SAGZM represents biotic and abiotic solids in the water column and abiotic solids in the sediments. New state variables for detritus solids in both water column and sediment compartments were added to the existing model to represent the complex particle dynamics. SAGZM/PCB model classified water column solids into biotic, detritus, and abiotic, whereas sediment solids into detritus and abiotic based on their organic carbon contents and origin. This type of representation of solids is important in terms of tracking particle bound hydrophobic organic contaminants (HOCs) such as PCBs that tend to adsorb onto the organic fraction of solids.
In SAGZM/PCB, the detritus solids are assumed to be produced by excretion, respiration, and decomposition of algae and zooplankton in water column, whereas, those are primarily produced from rejection and excretion of particles (as feces and pseudofeces) by zebra mussels in sediments. Furthermore, new state variables representing PCB concentrations in water column, sediments, biota, and zebra mussels were added to SAGZM to estimate the fate and transport of hydrophobic contaminants. In summary, this revised aquatic ecosystem modeling framework (SAGZM/PCB) couples eutrophication and toxic chemical models. Since there is no coherent temporal and spatial PCB data set available to calibrate the model, this study attempted to conduct a screening level simulation of PCB cycling in Saginaw Bay.
To realize the research goal, a detailed analysis of the fate and transport of PCBs and bioaccumulation in the lower pelagic food chain in Saginaw Bay ecosystem has been carried out. Along with screening level applications, a sensitivity analysis of various forcing functions (e.g phosphorus and PCB loading, zebra mussel density, hydrological forcing functions) and important processes for mass transfer (e.g selectivity of different food particles by zebra mussels, lipid content of mussels, octanol water partition coefficients) is presented. The diagnostic application included a component mass balance analysis of the various mass flows of PCBs in the system and effect of mussels on bioaccumulation in the lower pelagic food chain. A diagnostic analysis of the system with and without zebra mussels was conducted to understand the interactions among various ecosystem components relevant to the problem of PCB bioaccumulation in the lower pelagic food chain.
Information on the study site and its description, water quality issues of the Bay, available data sources, modeling results and calibration of the coupled phytoplankton-zebra mussel model are presented in the previous reports (LTI 1995, 1997). The information on zebra mussel densities and the bay segmentation scheme is repeated in this report for convenience.
Previously Saginaw Bay modeling work on eutrophication (Bierman and Dolan 1981, 1986a, 1986b), fate and transport of contaminants (Richardson et al. 1983), and foodweb bioaccumulation (Kandt et al. 1993) has been developed. So the baseline conditions immediately prior to mussels’ invasion are well-documented. The extensive areas of hard bottom, along with ideal temperature and food regimes made the Saginaw Bay as a suitable site for zebra mussel colonization. Saginaw Bay is one of the 43 Areas of Concern as designated by the International Joint Commission and has an important commercial and sport fishery that could be affected by zebra mussel invasion. Also, Saginaw Bay is rich with studies conducted during the post-zebra mussel invasion. The details of those studies are provided in volume 21(4) of the Journal of Great Lakes Research. Availability of various studies during pre- and post-zebra mussel invasion and large zebra mussel colonization (Nalepa and Wojcik 1993) made Saginaw Bay a unique site for investigating PCB cycling.
The zebra mussel, Dreissena polymorpha, is a recent invader of the North American Great Lakes ecosystem that was introduced into the Lake St. Clair ecosystem in the mid-1980s (Hebert et al. 1989; Griffiths et al. 1991). After the introduction, zebra mussels have established extensive populations in the littoral zones of the lakes and have caused major ecological changes in the Great Lakes. Those changes include: decline in cholorophyll a concentrations and phytoplankton densities (MacIssac et al. 1992; Holland 1993; Nicholls and Hopkins 1993; Leach 1993; Fahnenstiel et al. 1995a), increase in water clarity (Hebert et al. 1991; Leach 1993; Pillsbury and Lowe 1994; Skubinna et al. 1995), decrease in rotifer densities (MacIssac et al. 1995), alteration in nutrients (Johengen et al. 1995; Fahnenstiel et al. 1995a; Heath et al. 1995; Mellina et al. 1995; Arnott and Vanni 1996), increase in benthic fauna (Griffiths 1993; Pillsbury and Lowe 1994), elimination of native mussels in the Lake St. Clair (Nalepa 2000), shift in distribution of productivity (Fahnenstiel et al. 1995b) and appearance of nuisance microcystis (blue-green algae) blooms (Taylor 1995; Lavrentyev et al. 1995; Vanderploeg et al. 1996).
Recent technical reports (Nalepa et al. 1996; Johengen et al. 2000) describe the impact of zebra mussel on physical and chemical variables during 1991-1996 in Saginaw Bay. In another study, Nalepa et al. (1999) reported the impacts of zebra mussels on water quality of Saginaw Bay. The results illustrated that primary production, chlorophyll, and total phosphorus declined by 38%, 54%, and 47%, respectively. Typically a value of 0.2 - 0.8 d-1 has been reported as the filtration turnover rate (the theoretical time in days it would take for the mussel population to filter the entire water volume) for zebra mussels in Saginaw Bay (Fanslow et al. 1995).
Although there is a growing body of literature on ecosystem impacts of zebra mussels, the studies on the role of zebra mussels on contaminant cycling are limited. Fisher et al. (1993, 1994) investigated the uptake and elimination of hydrophobic chemicals by zebra mussels, finding relatively high bioconcentration potential. Bruner et al. (1994a and 1994b) have conducted laboratory uptake, elimination and feeding experiments for PCBs and polynuclear aromatic hydrocarbons (PAHs). Their work showed that the relative bioaccumulation and assimilation efficiency for two PCB congeners in mussels are dependent on total PCB concentration and phase distribution, dietary fraction of algae versus seston, and mussel body size and lipid content. Recent work focuses on the use of zebra mussels as bioaccumulators of contaminants (Endicott et al. 1998) and their use as biomonitors (Kraak et al. 1991; Secor et al. 1993; Morrison et al. 1995; Chevreuil et al.1996). So, there is a need to study the impact of zebra mussels on both cycling and bioaccumulation while assessing ecosystem response and effects due to the release of toxic chemicals.
Zebra mussels are responsible for many changes in the ecosystem due to their huge filtering capacity (Sprung and Rose 1988; Reeders and bij de Vaate 1990; Fanslow et al. 1995). The size range of particles filtered by zebra mussels, ca 1 mm (Sprung and Rose 1988) to 750 Ám (Ten Winkel and Davids 1982), includes most phytoplankton and many small invertebrates, including protozoans, rotifers and crustaceans (MacIsaac et al. 1991). The filtered material is either assimilated and incorporated into biomass, or deposited to the bottom sediments as feces and pseudofeces. The biological availability of PCBs sorbed to the feces and pseudofeces can serve as a reservoir of PCBs. The benthic invertibrates which feed on detritus and process contaminated sediments may also play a role in transferring PCB sorbed contaminants to the pelagic food chain (Landrum, 1988). Therefore, Dreissena could change the dynamics of nutrients and contaminants and biochemical energy flow by selectively altering different components of the food web, thereby affecting the bioaccumulation in the food web.
The cycling of contaminants in lakes is regulated by the transport of particles since the HOCs such as PCBs have a strong affinity for solid surfaces. The chemical partitioning of HOCs between the freely dissolved and particulate phase is considered to be a function of the fraction of organic carbon on the solids and octanol-water partition coefficient (Kow) of the chemical (Karickoff et al. 1979; Karickoff 1984). Zebra mussels are capable of filtering the majority of particulate material from waters in which they colonize, thereby affecting the cycling and transport of particle-bound contaminants.
The purpose of current study was to examine the role of zebra mussels on fate and transport of PCBs in an ecosystem. The specific objectives of this study were to:
- Develop an aquatic ecosystem modeling framework that couples eutrophication and toxic chemical models in presence of zebra mussels and configure the model to the Saginaw Bay system (SAGZM/PCB).
- Synthesize a quantitative understanding of the processes that influence the cycling of hydrophobic organic chemicals in Saginaw Bay.
- Conduct a screening level calibration of the model by comparison with available data on the system.
- Apply the model in a diagnostic mode to determine the impact of zebra mussels on PCB cycling and bioaccumulation in the pelagic lower food chain.
The diagnostic modeling analysis included the following:
- Calculate bioaccumulation of PCBs in zebra mussels.
- Assess the potential use of mussels as biomonitors of contaminants.
- Conduct a component mass balance analysis of the various mass fluxes of the PCBs in the system for both the scenarios in presence and absence of mussels.
- Compare the PCB mass stored in mussels with that in sediments.
- Perform tests on various hypotheses associated with ecosystem level interactions in presence of mussels.
- Conduct sensitivity analysis on both the forcing functions (e.g. phosphorus and PCB loading, zebra mussel densities) and important processes (e.g. zebra mussel selectivity on different type of particles, PCB partitioning, lipid contents of mussels).