Freshwater pearly mussel (Unionidae) survey of Otsego Lake following zebra mussel (Dreissena polymorpha) introduction Zach Piper1

ABSTRACT Surveys in Otsego Lake have documented pearly mussel (Family Unionidae) population changes over the last few decades. Beginning with the first comprehensive survey in 1969, six native species were known to inhabit Otsego Lake, New York. Recent surveys have documented considerable unionid declines in both species diversity and overall densities. Forty-two historical and additional sites were surveyed for pearly mussels in July and August 2016. This is the first comprehensive survey in Otsego Lake to find no live unionids. Additionally, increases in sedimentation and zebra mussel (Dreissena polymorpha) colonization on spent valves was documented. These and other factors are likely to have caused the native pearly mussels to be extirpated from Otsego Lake.

INTRODUCTION Freshwater pearly mussels of the order Unionida are among the richest and most diverse groups of fauna in Eastern North America (Strayer and Jirka 1997). Several species within this group are native to Otsego Lake, situated to the north of Cooperstown, New York. These animals live by filter feeding microorganisms, nutrients, and other particulates from the water while burrowed into the benthic substrate (Strayer and Jirka 1997). Unionids are a significant part of many freshwater ecosystems for their ability in regulating particle, toxin, and nutrient movement (Bowen et al. 1994; Strayer and Jirka 1997). Despite their apparent importance, pearly mussels have limited abilities to move along the substrate, making it difficult for these organisms to relocate to areas with more suitable environmental conditions (Strayer and Jirka 1997). This characteristic, combined with their sensitivity to invasive species, sediment accumulation, and various pollutants make pearly mussels useful bio-indicators for water quality and bio-monitors for environmental contaminants in Otsego Lake and other water bodies (Fuller 1974; Strayer and Jirka 1997). The longevity of pearly mussels also allows for greater accumulation of toxins to occur, providing information on environmental contamination (Carell et al. 1987; Pugsley et al. 1988). Previously, six species of unionids were known to exist in Otsego Lake: Alasmidonta undulata, Anodontoides ferussacianus, Elliptio complanata, Lampsilis radiata, Pygandon cataracta, and Strophitus undulatus (Harman 1970). Past surveys by Harman in 1969 (1970), Ferrara in 2000 (2001), and Caracciolo in 2013 (2014) have shown troubling declines in both overall abundance of mussels and also of species richness. This decline has been attributed in large part to the introduction of invasive zebra mussels (Dreissena polymorpha), first documented in 2007 (Harman 2008). They have since come to dominate the benthic communities of Otsego Lake (Horvath 2008; Waterfield 2009; Yoo et al. 2014). These organisms are particularly problematic for native bivalves due to their propensity to colonize the

1 SUNY Oneonta Biology Department intern, summer 2016. Current affiliation: SUNY Oneonta. posterior ends of other bivalves in groups of hundreds or even thousands, thus interfering with their capacity to feed and respire (Haag et al. 1993; Ricciardi et al. 1995). Massive native pearly mussel population losses since the introduction of Dreissena polymorpha have already been reported in the Great Lakes and are likely to continue to occur in Otsego Lake (Haag et al. 1993; Ricciardi et al. 1995). The decline in biomass and diversity of native bivalves in Otsego Lake between 1969 and 2013 is also hypothesized to be due to increased siltation as discussed by Harman (1970), Weir (1977), Ferrara (2001), and Caracciolo (2014). This study reports a survey of historical pearly mussel sites in Otsego Lake to determine current abundance and diversity of native bivalves. In addition, counts of zebra mussels colonizing bivalves and siltation levels were considered to conclude as probable causes of native bivalve losses.

METHODS Pearly mussels were surveyed in July and August 2016 via SCUBA diving. Sites surveyed included those from past studies (Harman 1970; Ferrara 2001; Caracciolo 2014) along with additional sites which were selected based on historical locations of live unionids, bottom composition, and proximity to other study locations. In total, 42 sites were utilized in this study to extensively cover most of Otsego Lake’s shoreline (Figure 1, Table 1). Divers followed a timed transect parallel to shore at each site based upon a chosen compass orientation for approximately ten minutes. We chose compass bearings for each site to maintain an approximate bottom depth of 1-3 meters. Transects at greater depths were taken if any obstructions (i.e. weedbeds, docks) interfered with surveying. Extra time was allotted for locations with ideal conditions for live unionids and evidence of recently deceased unionids. Any live unionids were to be either identified underwater or brought to the surface for further examination and then immediately returned to their original habitation with their posterior ends directed upwards. Search times were totaled and quantified as Catch Per Unit Effort (CPUE, the number of valves or pairs of valves collected per hour). Depth, GPS coordinates, and substrate composition were also recorded at each survey site.

Figure 1. Unionid survey sites on Otsego Lake, July and August 2016. Table 1. UTM coordinates of Otsego Lake unionid survey sites, July & August 2016.

Empty valves found at each site were collected for further examination. Valves were identified to species level, enumerated, and subsequently analyzed for zebra mussel and siltation coverage. Only zebra mussels attached to dead valves were removed and counted. Siltation levels were determined based on sediment accumulation characteristics described in Table 2. Table 2. Siltation level description used in Table 3 (from Box & Mossa 1999; Caracciolo 2014).

Average Siltation Description

Shell color and texture are easily seen, virtually no Low visible silt, sand or mud. Valve is not filled with silt

Majority of shell is clean of sediment, most of shell markings, color and texture are visible, some silt Medium accumulation inside empty valve. Could have effect on health of unionid Color and texture of shell are completely covered, majority of valve buried when found, empty valve High filled with sediment. Could cause severe health effects/death

RESULTS No living unionids were collected. A total of 220 dead unionids were collected during this study, with representatives from three of the six species that were native to Otsego Lake: 6 Elliptio complanata, 200.5 Lampsilis radiata, and 13.5 Pygandon cataracta. Total counts for each species, depth, bottom composition, number of zebra mussels attached, and siltation levels for each site surveyed are listed in Table 3. This study was the first comprehensive survey to not find live unionids in Otsego Lake. The most abundant dead unionid found was L. radiata, with E. complanata as the least abundant. Of the 42 number of sites, 31 were found to have unionid shells. The overall CPUE for the entirety of this study was 0.00 unionids/hour, a continued decrease from previous studies. In addition to the lack of live unionids, most empty valves were found with dozens of D. polymorpha or more attached via byssal threads. A total of 2,356.5 zebra mussels were removed from shells during this study. The most zebra mussels removed from a single shell was 193. Higher silt deposits on and inside unionid shells than previously recorded were also discovered throughout this study. These findings indicate that it is likely that Otsego Lake’s native pearly mussel populations have been extirpated.

Table 3. Unionid survey data collected in July & August 2016. Zebra mussel counts (# ZM) were only considered if attached to shells. See Table 2 for descriptions of Average Siltation (Avg. Siltation) classifications.

DISCUSSION Results from this study and previous publications show an alarming negative trend in the abundance and diversity of native unionid populations. This particular study alone suggests that living native unionids are no longer present in Otsego Lake, evident by the fact that no living individuals were discovered at any of the 42 sites surveyed. These data are supported by credible reports from divers diving in Otsego Lake since 2013 when the last live unionid was found (Lord 2016). A number of hypotheses have been developed to explain the exact cause for this decline. Past studies on Otsego Lake have proposed that poor habitat, increased siltation and invasive species are likely causes. Past unionid surveys conducted in Otsego Lake reported decreases in both abundance and diversity in native species in association with loss of suitable habitat (Ferrara 2001). Ferrara (2001) explored the possibility that lack of habitat combined with increases in siltation is associated with these mussel declines. The U-shape basin of Otsego Lake has been hypothesized to provide inadequate substrate due to steep, unstable slopes located along shorelines (Ferrara 2001). In addition, lentic environments are found to have smaller mussel populations than lotic waters likely due to poorer nutrient availability and lower productivity (Weir 1977). Weir (1977) described the bottom composition preferences and abundance of Otsego Lake’s native species. He concluded that L. radiata was the most abundant species, although our second most abundant species, P. cataracta, was listed as not abundant. P. cataracta was also reported to have a preference for gravel and fine sand (Weir 1977), a bottom composition that it was rarely found in throughout this survey. Other publications have stated that while pearly mussels may have varying substrate preferences, these are unreliable due to other possible factors that ultimately determine what substrates these organisms inhabit (Box and Mossa 1999; Downing et al 2000; Johnson et al. 2014). An increase in the amount of silt at various Otsego Lake survey sites has been reported as well (Ferrara 2001; Caracciolo 2014). Forty out of the forty-two sites we surveyed were described as having bottom compositions comprised partly of silt (Table 3). Levels of silt accumulation on and inside dead valves were also high in the majority of sites (Tables 2 & 3), however it should be noted that qualitative observations such as these are not the most reliable (Box and Mossa 1999; Green et al. 1989). Regardless, siltation has been documented as having detrimental effects on pearly mussels in numerous studies (Box and Mossa 1999; Downing et al. 2000; Johnson et al. 2014, Harman 2016). Fine particulates in the water column can be dealt with to an extent before a mussel’s health is threatened severely. Certain species such as E. complanata, L. radiata, and P. cataracta are more tolerable of silt (Downing et al. 2000) which could be why valves of these species were found in recent surveys. Increased sediment loading in rivers and other water bodies impairs normal feeding and respiration processes in mussels, although the exact effects are not fully understood (Box and Mossa 1999; Downing et al. 2000; Johnson et al. 2014; Harman 2016). The sources of increased siltation has been attributed to human activities such as logging and construction, however, the exact source of these microscopic particles are difficult to determine and cannot be confirmed with certainty (Johnson et al. 2014; Strayer and Jirka 1997). Zebra mussel data from this survey are concerning as well and likely associated with the observed Otsego Lake unionid declines (Caracciolo 2014). These invasive bivalves are known to attach to the posterior ends of native pearly mussels in large colonies, frustrating normal life processes (Haag et al. 1993; Ricciardi et al. 1995). Unionids infested with zebra mussels likely cannot draw in adequate nourishment with their siphons nearly or completely covered. Studies have also shown that zebra mussels may out-compete native unionids by filtering large amounts of algae, phytoplankton, and small zooplankton out of the water column (Parker et al. 1998; Strayer 2009). With regards to respiration, dissolved oxygen in the water column has been shown to significantly decrease in the presence of zebra mussels which could harm unionids further (Caraco et al. 2000; Effler et al. 2004; Strayer 2009). Pearly mussel declines have been observed throughout New York State within the last century (Strayer and Jirka 1997). Many unionid species are currently threatened and endangered largely due to human impacts and their subsequent effects such as pollution, sedimentation, loss of habitat, and introduction of invasive species. The loss of pearly mussels in Otsego Lake is troubling and should serve as a reminder that increased efforts need to be administered to protect remaining pearly mussel populations.

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