From Genes to Ecosystems: Integrative Approaches in Unionid Ecology II
Presiding: C Vaughn, Oklahoma Biological Survey; D E Spooner, University of Oklahoma
NB42C-01 INVITED 10:30h
A Reach Scale Comparison of Fluvial Geomorphological Conditions of Freshwater Mussel(Bivalvia: Unionoidea)Beds: Differences Between the Haves and Have Nots
The objective of this project was to quantify freshwater mussel physical habitat ranges in the lower White River, Arkansas, a non-wadable, sand-clay-gravel-bed river. Sediment and hydraulic regimes associated with six historic and six current mussel beds were examined over multiple stage events and between two contiguous river regions. The parameters being explored at each site includes velocity profiles, bedload transport, total suspended sediment (TSS), and discharge. Initial multivariate ANOVA analysis of mean transect values indicate statistically significant longitudinal differences for velocity, Froude number, bed velocity, and TSS both within a reach (1 km) and between regions (80 km); transect mean values for bedload collected during moderate flow weren't significantly different. Lateral differences are also of interest as White River mussel beds are typically located on the outside of bendways. Initial analysis of these data indicate the mid-channel section exhibits greater mean velocity, Froude number, and bed velocity than inside and outside channel sections across stages and regions. Lateral analysis of moderate flow bedload indicates outside and inside transport rates are similar, but significantly less than mid-channel rates. These trends are expected to continue throughout the remaining analysis, providing support for the flow refugia concept in the lower White River.
NB42C-02 10:45h
Conservation of the Special Concern Ouachita Creekshell [Villosa Arkansasensis (Lea 1852)]: Life Hisorty, Ecology, and Conservation Implications
The Ouachita Creekshell is a Species of Special Concern that is located in the Ouachita and Saline River Drainages in Arkansas and Oklahoma. For this paper two of the four objectives of this project will be discussed and are to: 1) conduct a status survey, and 2) conduct habitat assessments. Status surveys were conducted at 18 stations across the drainage systems and were based on previous status surveys from 1988. Status survey results of the 18 stations resulted in ranges of mussel richness from 0-16 and catch per unit effort ranges of 0.004 to 0.183 V. arkansasensis per minute. In comparing 1988 to 2004 populations, a Wilcoxon-Signed-Rank test revealed a significant change in creekshell populations between 1988 and 2004 with 14 stations losing, three gaining, and one staying the same. Habitat analysis revealed that 51% of the variability was explained by the first 3 PCA axes. Variables that were correlated with these axes included terrestrial vegetation coverage, boulder, bedrock ledge, and large woody debris coverage and percentage cobble, gravel and fines. The pattern in loss in population numbers between survey years needs to be closely followed and we anticipate that the habitat data will be insightful in understanding these losses.
NB42C-03 11:00h
Factors Controlling the Age Structure of Margaritifera Falcata in Two Northern California Streams
We investigated the population dynamics of Margaritifera falcata, in two northern California Coast Range rivers, the South Fork Eel and Navarro. Because they are long lived and relatively stationary, mussels provide insight into how environmental variables influence recruitment and mortality. We used fieldwork and an inverse theoretic modeling technique to measure and analyze age structures to identify factors that control population demographics. The analysis indicates there are distinct mortality rates for younger (< 20 years) and older (>20 years) mussels in the South Fork Eel. In addition, the analysis reveals that recruitment and mortality are both related to river hydrology. In particular, M. falcata recruit more successfully during low flow years than during high flow years. In the South Fork Eel River study area, healthy recruitment is occurring in which ~1,100 - 1,800 individuals are added to the population annually. In contrast, there has been little or no recruitment over the past 30 years in the Navarro River. This difference is likely a consequence of two non-mutually exclusive factors: land use history and host fish populations. By investigating the factors that shape local M. falcata population age structures, we obtained an understanding into how environmental variables and history influence mortality and recruitment.
NB42C-04 11:15h
Are Predators Limiting Zebra Mussel Colonization of Unionid Mussels in Great Lake Coastal Wetlands?
Although many native mollusc populations have been eliminated in the Laurentian Great Lakes by the exotic zebra mussel, recent surveys have found abundant unionid (Bivalvia: Unionidae) populations in some coastal wetlands. Unionid burrowing in soft sediments and predation by fish have been shown to reduce numbers of attached zebra mussels, and we tested these factors in a Lake Erie coastal wetland. In 2002, we held live unionids (Leptodea fragilis, Quadrula quadrula) and Pyganodon grandis shells in exclosures with wire mesh bottoms that were buried to sediment depths of either 5, 10, or 20 cm. After 2 months, numbers of attached dreissenids on unionids were significantly higher inside all exclosure treatments than outside exclosures. This indicated that either unionid burrowing was prevented in all sediment depth treatments or molluscivores were excluded by exclosures. In 2004, we measured dreissenid colonization on Q. quadrula and PVC plates in bottomless exclosures with different mesh sizes. After 6 months, dreissenid numbers on PVC plates and on Q. quadrula in 2.5 cm X 2.5 cm and 5 cm X 10 cm mesh exclosures were significantly higher than in open exclosures. These data suggest that molluscivores are important in limiting dreissenids in Great Lake coastal wetlands.
NB42C-05 INVITED 11:30h
Hudson River Unionids and Zebra Mussels: The Beginning of the End or the End of the Beginning?
The invasion of the Hudson River estuary by zebra mussels was followed by steep declines (77 to >99.7%) in populations of all species of native bivalves between 1992 and 1999. Body condition of all unionids, and growth and recruitment of young unionids also declined significantly. Declines in population size and body condition were correlated primarily with the filtration rate of the zebra mussel population, not with fouling of native bivalves by zebra mussels. Samples taken since 2000, however, have shown that populations of all 4 common native bivalves have stabilized or even recovered, although the zebra mussel population has not declined. The mechanisms underlying this apparent reversal of fortune are not clear: recruitment and growth of young mussels have showed limited recovery, but body condition of adults has not. We found no evidence that spatial refuges contributed to this reversal of population declines. Simple statistical models project now that native bivalves may persist at population densities about an order of magnitude below their pre-invasion densities.