William F. James

The interaction between water movement, sediment dynamics and submersed macrophytes

Water movement in freshwater and marine environments affects submersed macrophytes, which also mediate water movement. The result of this complex interaction also affects sediment dynamics in and around submersed macrophyte beds. This review defines known relationships and identifies areas that need additional research on the complex interactions among submersed macrophytes, water movement, and sediment dynamics. Four areas are addressed: (1) the effects of water movement on macrophytes, (2) the effects of macrophyte stands on water movement, (3) the effects of macrophyte beds on sedimentation within vegetated areas, and (4) the relationship between sediment resuspension and macrophytes. Water movement has a significant effect on macrophyte growth, typically stimulating both abundance and diversity of macrophytes at low to moderate velocities, but reducing growth at higher velocities. In turn, macrophyte beds reduce current velocities both within and adjacent to the beds, resulting in increased sedimentation and reduced turbidity. Reduced turbidity increases light availability to macrophytes, increasing their growth. Additionally, macrophytes affect the distribution, composition and particle size of sediments in both freshwater and marine environments. Therefore, establishment and persistence of macrophytes in both marine and freshwater environments provide important ecosystem services, including: (1) improving water quality; and (2) stabilizing sediments, reducing sediment resuspension, erosion and turbidity.

Sediment resuspension and light attenuation in Peoria Lake: can macrophytes improve water quality in this shallow system?

We examined sediment resuspension and light attenuation in relation to the potential for macrophytes to improve water quality conditions in Peoria Lake, Illinois (U.S.A.). The lake exhibited high total suspended solids (TSS) loading and retention of predominantly fine-grained particles in 2000. Large fetches along prevailing wind rose, coupled with shallow morphometry and sediment particles composed of >90% silt and clay resulted in frequent periods of sediment resuspension. As calculated (wave theory) shear stress increased above the critical shear stress (measured experimentally), turbidity increased substantially at a resuspension monitoring station. Resuspension model explorations suggested that establishment of submersed aquatic macrophytes could substantially reduce sediment resuspension in Peoria Lake. However, Kd is currently very high, while Secchi transparency low, at in-lake stations. Thus, in order to establish a persistent macrophyte population in the lake to control resuspension, the underwater light regime will have to improve quite dramatically.

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.

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.

Variations in the aluminum:phosphorus binding ratio and alum dosage considerations for Half Moon Lake, Wisconsin

The aluminum:phosphorus binding ratio (Al:P) is an important variable for estimating the Al dosage required to inactivate loosely bound and iron-bound P (redox-P) in sediment for internal P loading control in lakes. For shallow Half Moon Lake, Wisconsin, the Al:P ratio varied in a negative exponential pattern as a function of increasing redox-P concentration. While more Al was needed to inactivate higher concentrations of redox-P, inactivation was more efficient at higher redox-P. The Al:P ratio needed to bind 90% of the redox-P exceeded 150:1 for redox-P concentrations <0.2 mg/g and approached 20:1 for concentrations >2.0 mg/g. Competition for binding sites by other constituents in relation to redox-P concentration may be responsible for this pattern. Although organically bound P was not important in Half Moon Lake, it may be in other cases, and lake specific assays are recommended to determine the most appropriate Al dosage. Even then, slower processes of P release from labile organic P and vertical diffusion may not be addressed by higher Al dosages, and more research is warranted. Because redox-P varied horizontally as a result of lake bathymetry, variations in the Al:P ratio were considered for lake-wide alum dose calculation for Half Moon Lake. The estimated lake-wide average dosage of 115 g Al/m2 was high but similar to other recent effective treatments reported in the literature.

Biologically Labile and Refractory Phosphorus Loads from the Agriculturally-Managed Upper Eau Galle River Watershed, Wisconsin

Abstract Fractionation techniques were used to quantify various biologically labile (i.e., directly available for biological uptake or subject to recycling pathways) and refractory (i.e., biologically unavailable and subject to burial) particulate and soluble phosphorus (P) forms along the longitudinal axis of the agriculturally-managed Upper Eau Galle River watershed in west-central Wisconsin. P loading increased as a function of increasing distance from the rivers headwaters. However, areal P export rates were similar longitudinally, indicating a relatively homogeneous land-use mosaic throughout the watershed. P loads were composed of predominantly biologically labile constituents (i.e., 79%), with soluble P forms (i.e., soluble reactive and unreactive P) accounting for 49% and labile particulate P forms (i.e., loosely-bound PP, iron-bound PP, and labile organic/polyphosphate PP) accounting for 30% of the P load. Soluble P forms are either directly available for biological uptake or can be converted to available forms through enzymatic (i.e., alkaline phosphatase) reactions. Deposition and retention of loosely-bound and iron-bound PP in the receiving impoundment, Eau Galle Reservoir, can become an important source of internal P loading via eH and pH chemical reactions. Suspended solids loads also exhibited a high equilibrium P concentration (i.e., EPC > 0.10 mg L−1) that was similar to flow-weighted soluble reactive P concentrations in the river, suggesting equilibrium control of soluble P as loads entered the reservoir. The high EPC and a linear adsorption coefficient approaching 1000 L kg−1 indicated that binding sites of eroded soils in the runoff were enriched with P due to soil nutrient management. Our results indicated that transformations, transport, and fate of biologically labile PP, as well as soluble P, need to be considered in load reduction management to eutrophic receiving waters.

Alum:Redox-Sensitive Phosphorus Ratio Considerations and Uncertainties in the Estimation of Alum Dosage to Control Sediment Phosphorus

Abstract Alum dosage requirements to immobilize loosely-bound and iron-bound sediment phosphorus (P) fractions (i.e., redox-sensitive P fractions) in the surface sediments of eutrophic, Squaw Lake, Wisconsin, were determined using alum assay procedures developed by Rydin and Welch (1999). Since the lake exhibits a low buffering capacity (alkalinity = 25 mg Ca L−1), an alkalinity-based calculation could not be used to estimate alum dosage. Redox-sensitive sediment P fractions of surficial sediments, which represented 44% of the inorganic sediment P, were depleted by greater than 90% at an alum (as Al):redox-sensitive P binding ratio of ~ 100:1. Our results suggest that a higher dosage of alum, based on a higher alum:redox-sensitive P binding ratio requirement, is necessary to achieve effective control of sediment P in this lake. However, uncertainties still exist in the calculation of an alum dosage based on redox-sensitive sediment P concentration. More research is needed to validate optimal alum:redox-sensitive P binding ratios for use in sediment P-based alum dosage calculations. Criteria for estimating the layer of profundal sediment (i.e., the volume of redox-sensitive sediment P or the active layer of sediment contributing to diffusive P flux) to treat is also needed in order to estimate a cost-effective alum dosage for reducing internal P loading.