We sampled adult zebra mussels twice a summer (early June and late August) from 2016-2018 at five depths (1, 3, 5, 8, and 10m) along each of three transects running perpendicular to shore (A-C). Dominant substrates at transect A were rock at 1 m depth, sand at 3 and 5 m, and muck at 8 and 10 m. At transects B and C, sand was the dominant substrate at 1 and 3 m depth and muck was dominant at 5, 8, and 10m. Significant macrophyte growth was generally absent at all sites in June and occurred mostly at 1, 3, and 5 m sites only at transects A and C. Because most sites lacked hard substrate (rocks, logs, etc.) suitable for zebra mussel colonization, we also sampled five additional rocky 1m depth sites to represent prime zebra mussel habitat.
Using SCUBA, we removed all adult mussels from triplicate quadrats at each site, placing mussels in resealable plastic bags and processing them int the laboratory for counts, lengths, and widths. Quadrat sizes varied by sampling site and year from 1 m2 to 0.0625 m2 because sampling with large quadrats became infeasible as zebra mussel densities grew, but we standardized all sample counts to an abundance measure of individuals m-2. For samples containing more than 100 mussels, we subsampled in the laboratory by spreading mussels evenly across a gridded surface and counting and measuring individuals until enough grid cells were processed to reach 100 individuals. We then standardized that count by the total number of grid cells used for subsampling to estimate total number of mussels in the sample. We assigned modeled lengths and widths to the unmeasured individuals by randomly selecting measurements from that sample's pool of 100+ measured individuals with replacement. We modeled biomass (g, wet weight) for each individual based on lake-specific length-to-weight biometric conversion factors developed from 99 individuals of a range of sizes collected from Lake Mendota in 2018. We recorded mean zebra mussel abundance density (# m-2) and several versions of biomass estimates (g m-2) on the lake bottom at each site.
Though all metrics of estimate biomass were modeled, the methods for measuring these metrics on the individuals for which the length-to-weight biometric conversion factors were generated are as follows. Body size directional measurements of shell length and width were recorded for every specimen with the aid of callipers (0.01 mm). Following this, any excess water was removed from surfaces by drying the external shell with tissue paper. Further, using a scalpel blade and tweezers, excess water was removed from the mantle cavity by gently forcing bivalves to gape, taking care not to cut the adductor muscle or damage tissues. Using high-resolution scales, living-weight (lw) was obtained for each specimen. Then each specimen was fully opened, which in most cases involved cutting of the adductor muscles. To remove additional fluid from the mantle and other cavities, each specimen was then placed with the valve gape (flesh) facing downwards onto absorbent tissue, for ~5-10 minutes. A wet-weight (ww) was obtained for each specimen. Following this, the soft tissue was dissected from the shell, then both soft tissue and shell were dried together within an oven (60-72 degreeC) for ~48 hrs, or until they reached a constant weight. Specimens were cooled to room temperature in a desiccator before final weighing. A combined dry-weight (dw) was recorded, as were weights for the soft tissue and shell separately, i.e. shell free dry-weight (sfdw) and dry shell-weight (sw), respectively. To obtain an ash-free dry weight (afdw) the soft and hard tissue structures of specimens were incinerated (500–550 degreeC) together within a muffle furnace for 4–6 hrs. Afdw was then calculated for the entire specimen (soft tissue and shell) by subtracting the post-incineration weight from dw. Length-to-weight conversion factors for each biomass metric were created by estimating non-linear regression parameters for power law equations, i.e. biomass metric = a x lengthb.
