Use of Bivalve Shell Size Distribution as a Measure of Transient Anoxia in Heterogeneous Benthic Habitats Exposed to Persistent Currents

During the first life span of long-lived benthic mussels, about 12 years for Dreissena bugensis (QM) in the Laurentian Great Lake Michigan, community size-frequency spectra move through a predictable sequence. Median lengths rise through the first 15 mm to reproductive adult stage, when QM split into discernible cohorts for several years, then smear into mid-size 10-25 mm animals with annual spurts of abundance in the <4 mm size class after spawning. On mid-lake reef plateaus and offshore deeper sand and clay flats that are scoured and fed by bottom currents, mono-layer QM communities slowly increase in median length to an as-yet-future equilibrium. Of particular interest during the second life span (years 12-25) is information available in size distributions of empty shells from dead animals. With few or no natural predators 50-100 km offshore in 50-70 m of water depth, QM communities showed no empty shells for the first 10 years of colonization, and in the next two years only a very few empty shells, all large (>25 mm) were observed. The likely old-age death of these early colonizers extended the expected life span considerably. By this time, a decade of filtering, ecosystem engineering through carpeting growth, and feces excretion had changed the benthos from hardpan clay and coral carbonate to a sedimentary habitat, but not depositional in the typical sense. Following recovery of bottom grabs yielding black empty shells, camera-on-a-rope surveys showed that rock-strewn areas and even slight depressions in flat surfaces occasionally had evidence of anoxic pits. The size spectrum of these empty shell samples, devoid of live animals, was more of a normal distribution as for previous observations of growing populations. This indicated that anoxic conditions had occurred so rapidly that even quagga mussels could not escape. Until empty shells become a dominant component of the biogenic carbonate, these differences can be used to identify aberrant physical-biological events.