Biodiversity wanes in New York

Throttling invasive species by TJ Sinnott

Bulwark for the Great Lakes and Hudson River by P Gerrity

Stopping ballast water "stowaways" by D Pughiuc

Biological pollutants in the Great Lakes by EL Mills, KT Holeck

Water quality signatures and the zebra mussel invasion by DA Matthews, SW Effler

Zebra mussel population dynamics: Implications for water quality modeling by CL Lange, DR Opdyke, JC Powers

Bad seeds: an introduction to invasive plants by AD Halpern, CA Boesse, AE Altor

You can help stop the plant invasion

President's message by D Ellis

Executive director's message by P Cerro-Reehil

People and places

NYWEA calendar

Sponsors at 73d Annual Meeting

Spring 2001 — Vol. 31, No. 1

Zebra mussel population dynamics: Implications for water quality modeling

Figure 1. Mature zebra mussels (Dreissena polymorpha).

The Onondaga County Department of Drainage and Sanitation (OCDDS) is conducting an Ambient Monitoring Program (AMP) of Onondaga Lake and the Three Rivers System, including portions of the Seneca, Oneida, and Oswego Rivers. The AMP is part of an Amended Consent Judgment between Onondaga County, NYSDEC, and the Atlantic States Legal Foundation. The AMP program is designed to assess changes in lake and river system water quality in response to the scheduled upgrades and CSO abatement measures for the Syracuse Metropolitan Wastewater Treatment Plant (Metro).

Monitoring the Three Rivers System will establish baseline data. With those data, scientists can assess the diversion of all or a portion of the Metro effluent from its current location in the south basin of the Lake to the Seneca River. A calibrated and validated water quality model of the Three Rivers System will be the vehicle for this assessment.

The establishment of the zebra mussels in the system in the early 1990s has complicated the assessment. Hence, the AMP includes assessment of zebra mussel populations, and zebra mussels have been incorporated into the Three Rivers System water quality model.

About the bivalve

The zebra mussel (Dreissena polymorpha) is a freshwater epifaunal bivalve that is native to Eastern Europe (Figure 1). It was first documented in the Great Lakes in 1988 (Hebert and others 1989). They were probably introduced through ballast water released by a trans-Atlantic vessel.

Zebra mussels have a planktonic larval stage that is suspended in the water; thus, they can be carried long distances by currents as they develop. Female mussels can produce over a million young each year, and larval densities of over 1000/L of water have been observed. At the end of the larval stage the zebra mussels settle and attach to any available hard substrate, including aquatic vegetation, by a structure called a byssus.

Since no native freshwater mussel can attach to hard substrate, the introduced zebra mussel had an open niche to exploit. They rapidly increased their range and entered the inland waters of New York State by June 1990 and were first detected in the Seneca River at Baldwinsville in May 1991 (ESEERCO 1992). By 1993 a population density of ~49,000 mussels/m² was reported in a reach of the Seneca River (Effler and Siegfried 1994).

When present in sufficient numbers, zebra mussels can dramatically affect water quality. These effects are primarily caused by metabolic processes including respiration and filtering. A 20-mm long mussel can effectively filter over 280 mL/hr of water (Kryger and Riisgard 1988) and remove all seston (suspended particles) between 1 µ and the diameter of its inhalant siphon (approximately 250 µ) (Jorgeson and others 1984). Through these metabolic processes, zebra mussels can:

• Change nutrient cycling through decreases in the
concentration of phytoplankton, suspended matter, and dissolved oxygen
• Increase soluble phosphorus and ammonia nitrogen
(Arnott and Vanni 1996, James and others 1997, Heath and others 1995, Johengen and others 1995).

Procedure for the river study

In fall 1999, the OCDDS and Beak Consultants assessed the zebra mussel population in portions of the Three Rivers System as a part of the AMP. The objective was to determine the distribution and abundance of zebra mussels along a ~30-mi reach of the Seneca and Oneida Rivers—from just east of Cross Lake to near Schroeppel Island (Figure 2).
 
See Figure 2 in a new window. Move the window to view text simultaneously.

The initial task was to map the habitat of the river as related to potential colonization by zebra mussels. Reaches of the river that contained comparable contiguous habitat were mapped (see Roman numerals in Figure 2). These zones were referred to as zebra mussel habitat zones. Within each habitat zone, scuba divers used standardized techniques to collect samples of the substrate and zebra mussels present. Subsequent analysis determined the density, size distribution, and biomass of the zebra mussel populations in each habitat zone as well as on each substrate type. These data are supporting a water quality model of the Three Rivers System.
 
Learn the details of the sampling methodology.

Results of the distribution study

The petite Ponar sampling provided a far better measure of habitat than the sonar, recording only large differences in bottom depth, bottom regularity and substrate type. Consequently, the sonar data were used only to verify habitat breaks such as where significant changes in habitat occurred.

The 31.67-mi study area comprised nineteen habitat zones (Figure 2). The zones ranged in length from 0.48 mi to 4.5 mi and occupied a mean area of between 0.018 and 0.220 mi². Zebra mussels were present in 93 (49%) of the 189 samples analyzed.

The mean density of mussels in the project area was 1945 individuals/m². The mean density of zebra mussels ranged from 9 mussels/m² in Zone I (the original river channel around the State Ditch Cut) to 12,977 mussels/m² in Zone III (the State Ditch Cut) (Figure 3). The highest density of mussels in any individual sample was 43,937/m² in Zone III.
 
See Figure 3 in a new window.

Size structure of the zebra mussel population

Three different community structures were evident from an examination of the length frequency data:

• One community type was dominated by zebra
mussels that were <10 mm in length.
• Large individuals (>20 mm) dominated the second
community type.
• The third community type had a more balanced
size structure, which included a mix of mussels of different sizes and ages.

The extensive variation in the size distribution of the zebra mussels present among habitat zones in this relatively short river reach was striking and unexpected. The most interesting distributions were found in Zones I, II and III (including the State Ditch Cut) where the population was almost exclusively made up of individuals <10 mm. Such individuals generally are young-of-year mussels, which for this study were mussels that settled during 1999.

The lack of larger live mussels that would have settled before 1999 in this reach suggests that some catastrophic event occurred that killed or displaced the pre-existing population of mussels. Since the majority of successful zebra mussel settlement events in temperate fresh water occur between June and October of each year, the unimodal distribution of young-of-year mussels indicates that the event occurred after the 1998 year class had settled (that is, after fall 1998), but before most of the settlement in 1999.

Die-off

The presence of multiple year classes of mussels in Zone IV, located just downstream of this reach, indicated that whatever contributed to the die-off in the State Ditch Cut did not affect the population of mussels everywhere in the river. A similar die-off was reported to have occurred in the Cut in 1994 (Effler and others 1997). The presence of large numbers of byssusses (their attachment structures) from adult mussels carpeting the hard substrate in the Cut, and the rafts of large (>20 mm) shells just downstream indicated that catastrophic die-offs in the Cut are not annual events.

Adverse condition

A similar event also may have occurred in the Outlet to Onondaga Lake (Zone XIV). Over 87% of the mussels in Zone XIV were <10 mm, and only 8% were >20 mm. The cobbles and other hard substrate examined in the Outlet lacked the carpet of byssusses from detached adults, suggesting an annual low survivorship of adults in the Outlet or that no appreciable settlement had occurred in the Outlet before 1999.

In contrast, a disproportionate number of larger individuals were found in a reach midway between the State Ditch Cut and Baldwinsville (Zones VI, VII, and VIII) (Figure 2). In this reach, between 65% and 70% of the mussels were >20 mm and fewer than 11% were <10 mm. The mussel densities in this reach were generally high (up to 2357/m²) and were attached to large cobbles and boulders.

It was noticed that a thick coat of black anhydric silt covered the hard substrate in this reach. This coating was not observed in other zones where mussels were found. It is thought that this coat could limit the survival of the young-of-year mussels, which need a firm substrate to attach to. Whether this coating of silt is present in all years and the mechanism that allowed the successful establishment of the larger mussels has not been determined.

Population structures: 1999 and 2000

In fall 2000, Onondaga County and Beak resampled the State Ditch Cut and the Onondaga Lake Outlet to assess whether appreciable changes to the population structure of the mussels had occurred. In 2000 the mean density of zebra mussels in the State Ditch Cut and the outlet were comparable to 1999. The length distribution of mussels in the Cut were similar between years, which indicated that there was no appreciable survival of the 1999 year class. In the Outlet, however, a bimodal distribution was found with modes centered on 7-mm and 20-mm mussels in 2000 rather than the unimodal distribution centered on 6-mm mussels present in 1999.

Significance of the results

Noteworthy seasonal or annual changes in the size structure of the zebra mussel population in river reaches where high densities of mussels are present could lead to appreciable temporal differences in water quality related to zebra mussel physiology. For example, respiration rates and, therefore, dissolved oxygen consumption rates as well as ammonia and soluble phosphorus excretion rates vary with the size of the mussels. Consequently a population composed entirely of small individuals, as observed in the Cut in 1999, will have a different effect on water quality than a population of multiple age classes.

Water quality model

The zebra mussel population data described here were collected to support the development of a state-of-the-science water quality model for the Three Rivers System. An integrated modeling framework is being developed to assess the assimilative capacity of the System and to guide management decisions regarding the diversion of the Metro effluent from Onondaga Lake to the Seneca River.

Zebra mussels are present in sufficient quantities to affect the assimilative capacity of the Three Rivers System. Zebra mussels, therefore, are explicitly included in the model framework. (Other factors included are two-dimensional hydrodynamics, nutrient cycling, and phytoplankton dynamics.)

The customized zebra mussel submodel is being developed specifically for the Three Rivers System. Based upon the principles of bioenergetics, this zebra mussel submodel will simulate growth, respiration, filtration, excretion, and the nutrient fluxes associated with these processes. By integrating these processes into a predictive water quality model, the following questions will be addressed:

• What is the current assimilative capacity of the
System, and is it sufficient to receive diverted Metro effluent?
• What effects do zebra mussels have on the
assimilative capacity of the System under both measured population densities and potential future population densities?

Answers to these questions will guide management decisions regarding the diversion of Metro effluent to the Seneca River. This model may also be applied to assess total maximum daily loads for the system.

Onondaga County has undertaken this complex project using a cooperative process to incorporate comments from various stakeholders including USEPA and NYSDEC. The County wants a tool that can be used for a variety of purposes. In addition to using the water quality model in the decision process regarding the Metro discharge, the County will use the model to evaluate the effects of discharges from three other County-owned treatment facilities on the Three Rivers System. The model will be the first of a series of models developed for the Onondaga Lake Watershed.
 
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Cameron L. Lange is a senior environmental scientist with Beak Consultants Incorporated, Lancaster, NY. Daniel R. Opdyke, PhD is a project engineer with Quantitative Environmental Analysis, LLC, Syracuse. Jeanne C. Powers is a sanitary engineer for the Onondaga County Department of Drainage and Sanitation, Syracuse. Cameron L. Lange is corresponding author. Phone 716-759-1200.