Maynard H. Schaus

Biomass-Dependent Diet Shifts in Omnivorous Gizzard Shad: Implications for Growth, Food Web, and Ecosystem Effects

Abstract We examined diet patterns of omnivorous gizzard shad Dorosoma cepedianum in Acton Lake, Ohio, during 1994-1997 using a multiple stable isotope analysis to quantify the role of this species in the system. On most dates, zooplankton were relatively depleted in δ13C (about −30‰ to −25.5‰) compared with sediments (−25‰), permitting construction of a mixing model to determine the proportion of C derived from benthic detritus and from planktonic productivity. During periods of greater gizzard shad biomass (>35 kg/ha), gizzard shad of more than 35-mm standard length (SL) derived most of their C from sediment detritus. When gizzard shad biomass was low (<15 kg/ha), zooplankton biomass increased and all sizes of gizzard shad derived most of their C from zooplankton. Conventional gut analyses corroborated these findings. Zooplanktivorous age-0 gizzard shad grew at three or more times the rate of those that were detritivorous. Rapid age-0 growth led to high gizzard shad biomass, a decrease in large zooplank...

Water quality changes following nutrient loading reduction and biomanipulation in a large shallow subtropical lake, Lake Griffin, Florida, USA

Lake Griffin received discharges for decades from muck farms developed on former floodplain wetlands, leading to hypereutrophic conditions. Management actions included wetland restoration of farmland to reduce nutrient discharges, and harvesting of gizzard shad to remove nutrients in fish biomass and reduce nutrient recycling from sediments. Despite a reported susceptibility to wind-driven sediment resuspension, there have been substantial improvements in water quality in Lake Griffin, including decreases in nutrient concentrations, chlorophyll-a, and cyanobacterial biovolume, and increases in water transparency. Water quality improvements in Lake Griffin were substantially greater than occurred in ten comparison lakes. External nutrient load reduction was the primary factor contributing to water quality improvement, although there was evidence of an effect of shad harvesting, including correlations between shad catch per unit effort and nutrient concentrations, and an estimated effect of biomass removal and recycling reduction accounting for about 40% of the external load during the harvest period. Net production of total nitrogen in the lake was strongly related to external total phosphorus loading, indicating phosphorus limitation of nitrogen fixation. The response of Lake Griffin indicates that the combination of external nutrient load reduction and biomanipulation can result in sustained improvements in water quality in shallow subtropical lakes.

A global database of nitrogen and phosphorus excretion rates of aquatic animals

Animals can be important in modulating ecosystem-level nutrient cycling, although their importance varies greatly among species and ecosystems. Nutrient cycling rates of individual animals represent valuable data for testing the predictions of important frameworks such as the Metabolic Theory of Ecology (MTE) and ecological stoichiometry (ES). They also represent an important set of functional traits that may reflect both environmental and phylogenetic influences. Over the past two decades, studies of animal-mediated nutrient cycling have increased dramatically, especially in aquatic ecosystems. Here we present a global compilation of aquatic animal nutrient excretion rates. The dataset includes 10,534 observations from freshwater and marine animals of N and/or P excretion rates. These observations represent 491 species, including most aquatic phyla. Coverage varies greatly among phyla and other taxonomic levels. The dataset includes information on animal body size, ambient temperature, taxonomic affiliations, and animal body N:P. This data set was used to test predictions of MTE and ES, as described in Vanni and McIntyre (2016; Ecology DOI: 10.1002/ecy.1582).

Leg injuries and wound repair among cosmetid harvestmen (Arachnida, Opiliones, Laniatores).

Previous studies of leg injuries in harvestmen have focused on the fitness consequences for individuals that use autospasy (voluntary detachment of the leg) as a secondary defense mechanism. Leg damage among non‐autotomizing species of laniatorean harvestmen has not been investigated. Under laboratory conditions, we damaged femur IV of Cynorta marginalis and observed with scanning electron microscopy (SEM) the changes in these wounds over ten days. We also used SEM to examine leg damage from individuals of three species of cosmetid harvestmen that were collected in the field. On the basis of changes in the external surface of the hemolymph coagulum, we classified these wounds as fresh (coagulum forming), recent (coagulum with smooth surface), older (coagulum is scale‐like with visible cell fragments), and fully healed (scale replaced by new cuticle growth on the terminal stump). Our observations indicate that wound healing in harvestmen occurs in a manner comparable to that of other chelicerates. Leg injuries exhibited interspecific variation with respect to the overall frequency of leg wounds and the specific legs that were most commonly damaged. In addition, we measured walking and climbing speeds of adult C. marginalis and found that individuals with fresh injuries (lab‐induced) to femur IV walked at speeds significantly slower than uninjured adults or individuals collected from the field that had fully healed wounds to a single leg. J. Morphol. 278:73–88, 2017. ©© 2016 Wiley Periodicals,Inc.

Harvest-Induced Size Structure Shifts Alter Nutrient Release by a Population of Omnivorous Fish

AbstractFish body size can have important consequences for physiological factors, such as feeding, respiration, and nutrient excretion. Thus, the size structure of a fish population can greatly impact its role in lake nutrient cycles. We examined how shifts in body size influenced phosphorus cycling by Gizzard Shad Dorosoma cepedianum in Lake Dora, Florida, which underwent a size-selective fish harvest during 2005 and 2006. We combined a size-structured stock assessment model with excretion prediction coefficients to determine how shifts in population biomass and size distribution affected nutrient cycling. The harvest selectively removed larger Gizzard Shad, reducing the population biomass by 29.4% during the 2 years after removal, and shifted the population toward predominance by smaller size-classes. However, phosphorus excretion decreased by only 19.7% during this period because smaller fish excreted more P/g of body size. Three years following the harvest, biomass averaged 8.0% below the pre-harvest ...