Harry L. Eakin

Nutrient Regeneration by the Zebra Mussel (Dreissena polymorpha)

ABSTRACT Changes in suspended seston and soluble nutrients were examined as a function of zebra mussel (Dreissena polymorpha) density in flow-through sediment-water microcosms to determine impacts on nutrient recycling. Overall, zebra mussels removed > 70% of the suspended seston and > 80% of the total chlorophyll a input to the microcosms. Declines in organic nitrogen in microcosms were accompanied by density-dependent increases in nitrate-nitrite nitrogen. We suggest that micro flora associated with the sediments in the microcosms were transforming ammonium nitrogen, which is an excretory product of zebra mussels, into nitrate nitrogen. Declines in total phosphorus in the microcosms also coincided with density-dependent increases in the concentration of soluble reactive phosphorus and its rate of regeneration, suggesting enhanced nutrient cycling via excretion. Calculated rates of soluble phosphorus regeneration due to zebra mussel activity ranged from 0.3 to 2.5 mg m−2 d−1 at mussel densities ranging b...

Internal Phosphorus Loading in Lake Pepin, Upper Mississippi River

ABSTRACT We examined rates of internal phosphorus (P) loading from sediments in Lake Pepin, an impoundment on the upper Mississippi River. Laboratory-determined rates of P release from the sediment, measured using intact sediment core systems, were very high, with mean values ranging from 3.8 to 15 mg m−2 d−1 under oxic and anoxic conditions, respectively. Independently-determined Fickean diffusional fluxes (mean of 4.9 mg m−2 d−1), measured in situ under oxic conditions using sediment peepers, were not statistically different from those rates determined in the laboratory under similar temperature, pH, and oxygen conditions. The dominant inorganic P fractions in the sediments were NaOH-extractable P and HCl-extractable P. Under both oxic and anoxic conditions, significant correlations existed between rates of P release and the NH4Cl- and NaOH-extractable P fractions, suggesting mat loosely-bound and iron- and aluminum-bound sediment P contributed to P release from the sediments in this lake. Correlations ...

Phosphorus Budget and Management Strategies for an Urban Wisconsin Lake

ABSTRACT Multiple external and internal phosphorus (P) sources to an urban lake, Half Moon Lake in Wisconsin, were examined during the summer of 1999 in order to develop management strategies for effective P control and reversal of eutrophication (Trophic State Index=74). Internal recycling of P accounted for 80% of the summer P budget of the lake. Flux of P from the sediment accounted for most of the internal P loading (42% of total budget). However, decomposition of Potamogeton crispus and recycling of macrophyte P during the middle of the summer growing season, and P resuspension due to motor boat activity, accounted for 20% and 17% of the P budget, respectively, representing additional important sources to be controlled. In contrast, summer P loading via the watershed (storm sewers and precipitation) was much less. Using a water quality model (Bathtub), we found that reduction of internal P sources could substantially reduce by greater than 70% the high concentrations of algae in the lake (mean summer chlorophyll = 82 mg m−3). Suggested internal P control measures included a sediment chemical treatment to bind P, greater harvesting of P. crispus to reduce the macrophyte P pool at the time of senescence, and limiting motor boat activity when the lake is weakly stratified.

Filtration and Excretion by Zebra Mussels: Implications for Water Quality Impacts in Lake Pepin, Upper Mississippi River.

ABSTRACT Filtration and soluble nutrient excretion were examined over a range of zebra mussel (Dreissena polymorpha) shell lengths (range = 6 to 32 mm) in experimental laboratory systems and combined with in situ shell length frequency distribution and areal estimates of zebra mussel population density to make predictions of overall areal filtration and soluble nutrient excretion rates in Lake Pepin, upper Mississippi River, USA. Zebra mussels removed seston and excreted ammonia and soluble phosphorus in laboratory systems. When normalized with respect to ash-free dry mass (i.e., μg g−1 AFD mass d−1), smaller zebra mussels filtered seston and excreted soluble nutrients at a higher rate than larger zebra mussels. Although overall zebra mussel density is currently very low in Lake Pepin (~150 ind. M−2), lakewide chlorophyll filtration rates of ~ 7 mg m−2 d−1 were equivalent to chlorophyll loading into Lake Pepin via external sources and represented a turnover of 11 days for chlorophyll standing crop in the lake. Lakewide estimates of excretion of soluble phosphorus of ~ 3 mg m−2 d−1 by zebra mussels in Lake Pepin were comparable to internal phosphorus loading from anoxic sediments in other eutrophic freshwater aquatic systems. Our results suggest that zebra mussels are currently having a modest impact on phytoplankton dynamics and P and N recycling in this system.

Convective water exchanges during differential cooling and heating: implications for dissolved constituent transport

Convective water exchange patterns, determined from water temperature variations, were examined in the Minky Creek embayment of Guntersville Reservoir, Alabama (USA), during the month of September, 1990. During periods of differential cooling, cooler water originating from shallow stations moved as an underflow current toward the center of the embayment, while warmer water moved as an overflow current toward the shore. During periods of differential heating, convective exchange was much more shallow, occurring in the upper 3 m of the water column. As warmer water from shallow regions moved out as an overrflow current, this water was replaced by a return flow of cooler water originating from the pelagic epilimnion. Wind speed appeared to influence convective exchange patterns during differential heating phases by affecting the depth of the surface mixed layer. The potential for convective exchanges during periods of differential cooling and heating occurred on 54% and 74% of the days, respectively, in September. This high potential for horizontal water exchanges in Guntersville Reservoir has strong implications for the lateral transport of dissolved constituents.

Phosphorus recycling by zebra mussels in relation to density and food resource availability

Using flow-through microcosms, we examined phosphorus (P) recycling by zebra mussels under conditions of nearly constant food resource supply and varying zebra mussel population densities (600–5200 ind./m2). At all density levels, zebra mussels filtered substantial algae, measured as chlorophyll biomass. Because chlorophyll biomass inputs were low throughout the study, zebra mussel biomass-specific rates of chlorophyll filtration declined with increasing density, suggesting food resource limitation at the higher densities. We observed net total P export and high zebra mussel biomass-specific rates of P recycling over time in microcosms at high zebra mussel densities. In systems with a low zebra mussel density, net total P export did not occur over time. Our results suggest the occurrence of P remineralization by zebra mussels and net loss associated with emaciation during periods of temporary starvation. These findings have implications for P dynamics since zebra mussels can be subjected to periods of starvation over seasonal and annual time scales.

Labile and Refractory Forms of Phosphorus in Runoff of the Redwood River Basin, Minnesota

ABSTRACT Fractionation procedures and phosphorus (P) adsorption-desorption assays were used to delineate labile forms (i.e., subject to transformations which make them available for uptake by biota) and refractory forms (subject to permanent burial) of P in the runoff of the Redwood River basin, an agriculturally-dominated tributary of the Minnesota River. Over several storm periods monitored in 1999, 75% of the P load originating from the watershed was in labile form while only 25% was in refractory form. Labile particulate P forms included P loosely bound to suspended sediment (19%), P bound to iron (11%), and labile particulate organic P (14%). After deposition to receiving waters, the former two forms of labile particulate P can be transformed to soluble forms that are available to biota for uptake via eH and pH reactions and kinetic processes, while the latter form can be mineralized via decomposition processes. Labile soluble P forms included ortho-P (i.e., estimated as soluble reactive P) and soluble organic P (i.e., estimated as soluble unreactive P). Our results indicate that there is a need to identify loading of various labile and refractory P pools for incorporation into watershed models to develop better predictive capabilities for examining P sources and fates, and impacts on the eutrophication of receiving waters, under different management and operational scenarios.