Improve Water Clarity Research Articles

Seasonal acclimation in metabolism reduces light requirements of eelgrass (Zostera marina)

We investigated the ability of eelgrass (Zostera marina) to adjust light requirements to seasonal changes in temperature, light and nutrient conditions through changes in metabolism, pigment and nutrient content. In agreement with expectations we found that rates of respiration and light saturated photosynthesis of summer acclimated plants peaked at higher temperatures (5°C and 2°C higher, respectively), and were lower than of winter acclimated plants, both at sub- and supra-optimal temperatures. Moreover respiration rates were generally more sensitive to increasing temperatures than photosynthetic rates, especially so for cold acclimated plants in February (36% higher Q10-values). These changes were accompanied by a reduction in chlorophyll a and nitrogen concentrations in leaves by 35% and 60% respectively from February to August. The critical light requirement (EC) of Z. marina to maintain a positive carbon balance increased exponentially with increasing temperature but less so for summer-acclimated than for winter-acclimated plants. However, combining EC vs temperature models for whole-plants with data on daily light availability showed that seasonal acclimation in metabolism increased the annual period, when light requirements were meet at the 2–5m depth interval, by 32–66days. Hence, acclimation is an important mechanism allowing eelgrass to grow faster and penetrate to deeper waters. Critical depth limits estimated for different combinations of summer temperatures and water clarity in a future climate scenario, suggested that expected increases in temperature and nutrient run-off have synergistic negative effects, especially in clear waters, stressing the importance of continued efforts to improve water clarity of coastal waters.

Using invertebrate remains and pigments in the sediment to infer changes in trophic structure after fish introduction in Lake Fogo: a crater lake in the Azores

Fish introduction may have marked effects on the trophic dynamics and ecological state of former fishless lakes, but due to scarcity of historical data this can seldom be documented. We used remains of cladoceran, chironomid and pigment assemblages in the sediment archive to unravel the effect of introduction of carp (Cyprinus carpio), rainbow trout (Oncorhynchus mykiss) and a cyprinid (Chondrostoma oligolepis) in Lake Fogo, the Azores (Portugal). The stratigraphical record showed two major shifts in community assemblage coinciding with the time of introduction of carp (AD ca. 1890) and trout (AD 1941), respectively. Carp introduction was followed by an abrupt and major decline in the abundance of chironomids, a shift in the cladoceran community from a benthic to a more pelagic dominated community, and Daphnia size was significantly reduced. Pigment assemblages also indicated a shift from a benthic to a pelagic dominated ecosystem, as cryptophytes became markedly more abundant at the expense of benthic diatoms. Trout introduction was followed by a return to a more benthic cladoceran and benthic algae (pigments) dominated state, which we attribute to trout predation on carp leading to improved water clarity. A steady increase in the abundance of pigments and cladoceran remains followed, suggesting enhanced productivity, which may be attributed to enhanced atmospheric nitrogen deposition and introduction of C. oligolepis. We conclude that fish introduction has profoundly altered the trophic dynamics and the relative importance of benthic and pelagic production in this species poor and natural fishless lake in the Azores, and likely in most others lakes at the archipelago islands as fish stocking has been a widespread practice.

Impact of the removal of gizzard shad (Dorosoma cepedianum) on nutrient cycles in Lake Apopka, Florida

Summary1. The St. Johns River Water Management District removed over 5.4 million kg of gizzard shad (Dorosoma cepedianum) from Lake Apopka, FL during 1993–2005, as a means of reducing lake phosphorus and phytoplankton concentrations and improving water clarity. Other steps included reduction of external nutrient inputs and operation of a treatment wetland. We measured nutrient excretion by Lake Apopka gizzard shad to quantify the nutrient effect of this biomanipulation.2. Both N and P excretion were significantly affected by fish body mass and temperature. Larger fish had lower mass‐specific rates of excretion than smaller fish.3. High water temperature increased P excretion to a much greater extent than N, resulting in a low N : P of nutrient excretion in midsummer. The N : P of excretion was lower than has been observed in other systems, probably because of higher water temperature.4. Removal of gizzard shad >200 g prevented the annual release of 45 800 kg N year−1 (3.46 kg N ha−1 year−1) and 7700 kg P year−1 (0.62 kg P ha−1 year−1) on average. The actual impact on the P cycle varied substantially from year to year (range 7900–78 800 kg N year−1; 1200–14 800 kg P year−1), primarily because of fluctuations in the catch.5. On an annual basis, the P directly removed in fish tissues was similar to that removed by the treatment wetland. The P excretion prevented by the removal of fish was approximately 20% of the reduction in external P loading achieved during 1993–2005.6. In the short term, most of the P demand of planktonic primary producers is met through recycling of P, which greatly exceeds external P loading. Depending on population biomass, phosphorus excretion by the resident gizzard shad population was similar in magnitude to the P release by diffusive flux from the sediments.

Cladoceran community responses to biomanipulation and re‐oligotrophication in Lake Vesijärvi, Finland, as inferred from remains in annually laminated sediment

Summary1. We studied the role of zooplankton in biomanipulation and the subsequent recovery phase in the Enonselkä basin of Lake Vesijärvi, using subfossil cladocerans in annually laminated sediment. Measures to restore the Enonselkä basin included reduction in external nutrient loading and mass removal of plankti‐ and benthivorous fish. Water clarity increased and the lake changed from a eutrophic to a mesotrophic state. However, some signs of increased turbidity were observed after 5–10 years of successful recovery.2. Annual laminae in a freeze core sample were identified and sliced, based on the seasonal succession of diatoms. Cladoceran remains and rotifer eggs were counted, and Daphnia ephippia and Eubosmina and Bosmina ephippia and carapaces were measured. Annual changes in pelagic species composition were studied with principal component analysis. Individual species abundance, size measurements and various cladoceran‐based indices or ratios (commonly used to reconstruct changes in trophic state and fish predation) were tested for change between four distinct periods: I (1985–1988) dense fish stocks, poor water quality; II (1989–1992) fish removal; III (1993–1997) low fish density, improved water quality; IV (1998–2002) slightly increased fish density and poorer water quality.3. After the removal of fish, the mean size of Daphnia ephippia and Eubosmina crassicornis ephippia and carapaces increased significantly. In contrast, the percentage of Daphnia did not increase. When based on ephippia, the ratio Daphnia/(Daphnia + E. crassicornis) increased, but the interpretation was obscured by the tolerance of fish predation by small Daphnia and by the fact that bosminids were the preferred food of roach. Moreover, ephippial production by E. crassicornis decreased in recent years.4. The abundance of Diaphanosoma brachyurum and Limnosida frontosa increased significantly after the fish population was reduced, while that of Ceriodaphnia and rotifers decreased.5. The expanding littoral vegetation along with improved water clarity was clearly reflected in the concentration of littoral species in the deep sediment core. The species diversity index for the entire subfossil community also increased.6. The period of faltering recovery was characterised by greater interannual variability and an increased percentage of rotifers. Nevertheless, the mean sizes of Daphnia ephippia and E. crassicornis ephippia and carapaces indicated a low density of fish. The deteriorating water quality was apparently related to multiple stressors in the catchment after rehabilitation, such as intensified lakeshore building, as well as to exceptional weather conditions, challenging the management methods in use.

Impacts of changes in water quality on recreation behavior and benefits in Finland

The implementation of the European Union Water Framework Directive (WFD) requires nationally generalizable estimates of the benefits of protecting inland and coastal waters. As an alternative to benefit transfers and meta-analyses, we utilize national recreation inventory data combined with water quality data to model recreation participation and estimate the benefits of water quality improvements. Using hurdle models, we analyze the association of water clarity in individuals' home municipalities with the three most common water recreation activities – swimming, fishing and boating. The results show no effect on boating, but improved water clarity would increase the frequency of close-to-home swimming and fishing, as well as the number of fishers. Furthermore, to value the potential benefits of the WFD, we estimate the consumer surplus of a water recreation day using a travel cost approach. A water policy scenario with a 1-m improvement in water clarity for both inland and coastal waters indicates that the consumer surplus would increase 6% for swimmers and 15% for fishers. In contrast to previously estimated abatement costs to improve water quality, net benefits could turn out to be positive. Our study is a promising example of applying existing national recreation inventory data to estimate the benefits of water quality improvements for the purposes of the WFD.

Effect of Lake Management Efforts on the Trophic State of a Subtropical Shallow Lake in Lakeland, Florida, USA

For more than a decade, Lakeland, FL, has invested in restoring its urban Lake Hollingsworth from a hypereutrophic state to its natural eutrophic state. The lake bottom was dredged of nearly 2 million m3 of accumulated organic sediments, and treatment wetlands, storm water curb inlet strainers, and a storm water baffle box were installed within the lake’s catchment area to reduce the loading of dirt, leaves, and trash to the lake. After dredging ceased, the lake was dosed one time with alum to improve water clarity and reduce phosphorus recycling from its sediments. Water quality surrogates for algal biomass— Secchi disk transparency and water column total nitrogen, total phosphorus, and chlorophyll-α concentrations— were reviewed to assess Lakeland’s progress towards its goal. In the years since dredging has stopped, algal biomass concentration in Lake Hollingsworth has significantly declined. Even with these improvements, however, the lake still remains hypereutrophic.

Potential toxicity concerns from chemical coagulation treatment of stormwater in the Tahoe basin, California, USA

Coagulant dosing of stormwater runoff with polyaluminum chlorides (PACs) is used in numerous waterbodies to improve water clarity, but the potential risks of PACs to aquatic organisms in Lake Tahoe, California are not fully understood. To assess these risks, the USEPA 3-species toxicity test and a non-standard fish test using Japanese medaka ( Oryzias latipes) were used to determine the toxicity of PAC-treated and non-treated stormwater samples to aquatic species. Stormwater samples were collected from three sites representing runoff from different urbanized areas in May 2004; samples received coagulant dosing using three different coagulants (JC1720, PAX-XL9, Sumalchlor50) at levels optimized with jar testing. Raw stormwaters were toxic to algae and fathead minnows (mortality). Treatment with coagulants increased toxicity to zooplankton (reproduction) and had no consistent effects on the other toxicity metrics.

Investigating public preferences for managing Lake Champlain using a choice experiment

The Lake Champlain Basin in Vermont and New York, USA and Quebec, Canada includes a large lake and watershed with complex management issues. A transboundary comprehensive management plan prepared for the lake includes 11 goals across many issue areas. We developed a choice experiment to examine public preferences for alternative Lake Champlain management scenarios across these issue areas. Five ecosystem attributes (water clarity–algae blooms, public beach closures, land use change, fish consumption advisories and the spread of water chestnut, an invasive plant) were varied across three levels and arrayed into paired comparisons following an orthogonal fractional factorial design. Two thousand questionnaires were distributed to basin residents, each including nine paired comparisons that required trading off two, three or four attributes. Completed surveys yielded 6541 responses which were analyzed using binary logistic regression. The results showed that although water clarity and beach closures were important, safe fish consumption was the strongest predictor of choice. Land use pattern and water chestnut distribution were weaker but also significant predictors, with respondents preferring less land development and preservation of the agricultural landscape. Current management efforts in the Lake Champlain Basin are heavily weighted toward improving water clarity by reducing phosphorus pollution. Our results suggest that safe fish consumption warrants additional management attention. Because choice experiments provide information that is much richer than the simple categorical judgments more commonly used in surveys, they can provide managers with information about tradeoffs that could be used to enhance public support and maximize the social benefits of an ecosystem management program.

Response of submersed aquatic vegetation (SAV) in Lake Pontchartrain, Louisiana to the 1997–2001 El Niño Southern Oscillation shifts

Lake Pontchartrain is a large, shallow, low salinity estuary north of New Orleans, Louisiana. It is a water quality impaired system with restoration efforts in progress. One restoration goal is the reestablishment of historic submersed aquatic vegetation (SAV;Vallisneria americana Michx. andRuppia maritima L.), which has been in a state of decline since first studied in 1953. Annual SAV surveys and monthly water quality monitoring were conducted at four to five sites from 1996 through 2003 to evaluate trends and determine the causes of SAV change. We found a rapid increase in the distribution and abundance ofR. maritima in 1999 that persisted through 2002. An El Nino Southern Oscillation shift occurred between 1997 and 2001, which produced a drought in southern Louisiana as an ancillary effect of La Nina. This study was conducted to investigate causal links between the El Nino to La Nina climate phase shift and SAV change. We found that salinity and water clarity increased during La Nina. Increased water clarity produced a rapid increase in the euryhaline speciesR. maritima in deeper water and at historic sites where SAV had not been found since 1953. As salinity increased, the freshwater speciesV. americana andMyriophyllum spicatum L. declined, andNajas guadalupensis (Spreng.) Magnus andPotamogeton perfoliatus L. disappeared. In 2003, after the La Nina phase, salinity and water clarity decreased,R. maritima decreased, and the freshwater species increased, butP. perfoliatus was still absent. We found that salinity controlled SAV species composition, and water clarity controlled SAV colonization depth (Zcol=2.3/Kd). Our study demonstrated that climatic shifts cause cyclic changes in Lake Pontchartrain SAV and that restoration could be accomplished by improving water clarity. Due to the sensitivity of SAV to environmental change, similar responses to short-term and long-term climate changes should occur in other estuarine systems.

Modeling seagrass density and distribution in response to changes in turbidity stemming from bivalve filtration and seagrass sediment stabilization

In many areas of the North American mid-Atlantic coast, seagrass beds are either in decline or have disappeared due, in part, to high turbidity that reduces the light reaching the plant surface. Because of this reduction in the areal extent of seagrass beds there has been a concomitant diminishment in dampening of water movement (waves and currents) and sediment stabilization. Due to ongoing declines in stocks of suspension-feeding eastern oysters (Crassostrea virginica) in the same region, their feeding activity, which normally serves to improve water clarity, has been sharply reduced. We developed and parameterized a simple model to calculate how changes in the balance between sediment sources (wave-induced resuspension) and sinks (bivalve filtration, sedimentation within seagrass beds) regulate turbidity. Changes in turbidity were used to predict the light available for seagrass photosynthesis and the amount of carbon available for shoot growth. We parameterized this model using published observations and data collected specifically for this purpose. The model predicted that when sediments were resuspended, the presence of even quite modest levels of eastern oysters (25 g dry tissue weight m−2) distributed uniformly throughout the modeled domain, reduced suspended sediment concentrations by nearly an order of magnitude. This increased water clarity, the depth to which seagrasses were predicted to grow. Because hard clams (Mercenaria mercenaria) had a much lower weight-specific filtration rate than eastern oysters; their influence on reducing turbidity was much less than oysters. Seagrasses, once established with sufficiently high densities (>1,000 shoots m−2), damped waves, thereby reducing sediment resuspension and improving light conditions. This stabilizing effect was minor compared to the influence of uniformly distributed eastern oysters on water clarity. Our model predicted that restoration of eastern oysters has the potential to reduce turbidity in shallow estuaries, such as Chesapeake Bay, and facilitate ongoing efforts to restore seagrasses. This model included several simplifiying assumptions, including that oysters were uniformly distributed rather than aggregated into offshore reefs and that oyster feces were not resuspended.

Long-term Dreissenid Impacts on Water Clarity in Lake Erie

Since shortly after their introduction, dreissenid mussels have been thought to have improved water clarity in Lake Erie, particularly in the western basin. However, long-term monitoring (1982–2004) has found no evidence of persistent, basin-wide increases in water clarity in either the western or the central basin of Lake Erie since the Dreissena invasion. In fact, spring water clarity in both of those basins has exhibited statistically significant declines in the post-dreissenid period. In contrast, chlorophyll a levels in the western basin have declined by about 50% since the Dreissena invasion during both spring and summer. The discrepancy in the responses of water clarity and chlorophyll a is probably a consequence of both the large sediment loads entering the western basin and resuspension of unassimilated non-algal particulates. In the eastern basin, spring transparency has increased substantially and turbidity has decreased since Dreissena colonization, in spite of the much greater depth of this basin. This is probably due to higher mussel densities and the lack of major sources of turbidity in that basin. Summer turbidity has also decreased markedly in the eastern basin, although thermal stratification during this period would probably preclude direct filtration effects. Instead, we hypothesize that reductions in whiting events due to calcium uptake by dreissenids have contributed to the summer decreases in turbidity seen in the eastern basin.

Carp Exclusion, Food-web Interactions, and the Restoration of Cootes Paradise Marsh

Carp were excluded from Cootes Paradise Marsh (Lake Ontario) in 1997 in order to improve water clarity and promote submerged plant growth. On average, turbidity at open water and vegetated areas was reduced by 40 and 60 percent, respectively, following carp exclusion. However, responses by plants and other trophic levels have been both spatially and temporally variable due in part to annual variation in environmental conditions and fish-zooplankton interactions. In 1997, an unusually cool spring delayed the migration of spawning fish, including a usually abundant planktivore population, into the marsh. This had three main effects: 1) large Daphnia were released from predation in early summer and reached unprecedented numbers (530 Daphnia/L) in open water areas, 2) despite the lack of vegetated marsh habitat, larval fish production was high due to reduced predation and abundant zooplankton prey, and 3) zooplankton grazing initiated a spring clear-water phase which, together with carp exclusion, promoted submerged plant growth in shallow areas previously devoid of vegetation. These newly vegetated areas showed the greatest improvements in clarity and macrophyte growth in the first 2 years following exclusion. Currently, however, the future of the biomanipulation remains uncertain, due in part to natural climatic confounding factors, and further remedial actions will be required before this wetland represents a stable, clear-water, macrophyte dominated state.

A novel technology to improve drinking water quality: a microbiological evaluation of in-home flocculation and chlorination in rural Guatemala

An estimated 1 billion persons in low-income countries do not have access to improved drinking water. Chlorine, a useful water treatment agent, is less effective in turbid water, and lacks a visible effect, limiting its acceptability. A product incorporating precipitation, coagulation, flocculation, and chlorination technology (combined product) to reduce microbial, organic and heavy metal contaminants in water was evaluated. The combined product's microbiological efficacy in Guatemalan villagers' households was evaluated. One hundred randomly selected households from four neighboring Guatemalan villages were enrolled. Three groups received the combined product and either the Centers for Disease Control (CDC) water storage vessel, a covered bucket with spigot, or no vessel. One group received chlorine bleach and the CDC water storage vessel, and one group no intervention. Household water samples were collected for 4 weeks and Escherichia coli, chlorine, and turbidity levels were measured. Potable water was defined as having less than one E. coli per 100 ml. Eight (8%) baseline water samples were potable. Follow-up water samples were more likely to be potable than control samples (combined product and traditional vessel 83%; combined product and CDC vessel 92%; combined product and covered bucket with spigot 93%; chlorine and CDC vessel 92%; versus control 5%). Among combined product users, 98% reported improved water clarity compared with 45% of chlorine bleach users (p < 0.0001). The combined product technology improved water potability as effectively as chlorine bleach; improved water clarity could motivate more persons to effectively treat their water.

Age, Growth, and Gonadal Characteristics of Adult Bighead Carp, Hypophthalmichthys nobilis, in the Lower Missouri River

Bighead carp were introduced into Arkansas in 1973 to improve water clarity in production ponds. Bighead carp subsequently escaped aquaculture facilities in the early 1980's and dispersed into the Mississippi and Missouri rivers. The first documentation of bighead carp reproduction in the Mississippi River system was in 1989. The population has increased in the Missouri River as is evident in their increased proportion in the commercial harvest since 1990. The effect of this exotic planktivore on native ecosystems of the U.S. has not been examined. Basic biological data on bighead carp Hypophthalmichthys nobilis in the Missouri River are needed to predict potential ecological problems and provide a foundation for manipulative studies. The objectives of this study were to assess age, growth, and gonadal characteristics of bighead carp in the Missouri River. Adult bighead carp in our sample varied from age 3 to age 7 and length varied from 475 to 1050 mm. There was a large variation in length at age, and overall bighead carp exhibited fast growth. For example, mean back-calculated length at age 3 was 556 mm. The sample was dominated by bighead carp from the 1994 year class. There was no difference in gonad development (i.e., gonadal somatic index, egg diameter) between winter and spring samples. Length of male bighead carp and GSI were not significantly correlated; however, females exhibited a positive linear relationship between length and GSI. In each ovary, egg diameter frequencies exhibited a bimodal distribution, indicating protracted spawning. Mean fecundity was 226 213, with a maximum fecundity of 769 964. Bighead carp in the Missouri River have similar life history characteristics to Asian and European populations. They have become well established in the Missouri River and it is likely that dispersal and population density will increase.

Predicted impact of zebra mussel (Dreissena polymorpha) invasion on water clarity in Lake Mendota

Lake Mendota, which is plagued by periodic cyanobacterial blooms, is typical of many lakes in the Laurentian Great Lakes region that are vulnerable to zebra mussels (Dreissena polymorpha) but have yet to be invaded. We coupled removal estimates with chlorophyll production estimates in a dynamic model to predict the likely impact of mussel-mediated removal of phytoplankton on water clarity across a range of hypothetical zebra mussel densities. Models were fit to chlorophyll and temperature data collected biweekly from Lake Mendota during 1977-1993. When we assumed daily epilimnetic circulation, the percentage of days when the chlorophyll concentration exceeded 50 µg·L-1 was decreased threefold at mussel densities as low as 1000 mussels·m-2. When we assumed less frequent epilimnetic circulation, the density of mussels required to substantially improve water clarity increased dramatically. We predict that zebra mussel invasion would lead to increased water clarity in Lake Mendota. Cyanobacterial blooms would be reduced but not eliminated. Negative impacts on other lake processes following zebra mussel invasion could outweigh the benefits of lower phytoplankton concentrations.

Predicted impact of zebra mussel (&lt;i&gt;Dreissena polymorpha&lt;/i&gt;) invasion on water clarity in Lake Mendota

Lake Mendota, which is plagued by periodic cyanobacterial blooms, is typical of many lakes in the Lauren- tian Great Lakes region that are vulnerable to zebra mussels (Dreissena polymorpha) but have yet to be invaded. We coupled removal estimates with chlorophyll production estimates in a dynamic model to predict the likely impact of mussel-mediated removal of phytoplankton on water clarity across a range of hypothetical zebra mussel densities. Models were fit to chlorophyll and temperature data collected biweekly from Lake Mendota during 1977-1993. When we assumed daily epilimnetic circulation, the percentage of days when the chlorophyll concentration exceeded 50 mg·L -1 was decreased threefold at mussel densities as low as 1000 mussels·m -2 . When we assumed less frequent epilimnetic circulation, the density of mussels required to substantially improve water clarity increased dramatically. We predict that zebra mussel invasion would lead to increased water clarity in Lake Mendota. Cyanobacterial blooms would be re- duced but not eliminated. Negative impacts on other lake processes following zebra mussel invasion could outweigh the benefits of lower phytoplankton concentrations.

Algal responses to dissolved organic carbon loss and pH decline during whole‐lakeacidification: Evidence from paleolimnology

Fossil pigment analyses and 19 year‐long historical records were used to quantify whole‐lake algal response to changes in optical and chemical properties following experimental acidification of Lake 302 with H2SO4 (south basin, 302S; 1981—1989) or HNO3 (north basin, 302N; 1982–1986) and HCl (1987–1989). Undisturbed sediments were collected by freeze‐coring, sectioned in approximately annual intervals, and analyzed for fossil carotenoids, chlorophylls, and derivatives by high performance liquid chromatography. Concentrations of fucoxanthin (diatoms, chrysophytes, some dinoflagellates) were correlated with algal standing crop (r2 = 0.67, P &lt; 0.05; 1978–1989) and increased 6‐fold following acidification of Lake 302S with H2SO4 from pH 6.6 to 5.0, consistent with observed reductions in dissolved organic carbon (DOC) from 7 to 4.5 mg liter−1, improved water clarity, and increased biomass of deep‐water chrysophytes. However, fucoxanthin concentrations declined to baseline values in sediments from 1988 to 1990, concomitant with severe acidification to pH 4.5, continued DOC loss (&lt;1.5 mg liter−1) and an estimated 8‐fold increase in the penetration of UVb radiation (UVR‐b). Increased penetration of ultraviolet radiation (UVR) was recorded also by increased relative abundance of pigments characteristic of UVR‐transparent environments. In contrast, pigments from green algae (Chl b, pheophytin b, lutein‐zeaxanthin) doubled during acidification with H2SO4, while those from cryptophytes (alloxanthin) were unaffected and diatoxanthin from diatoms declined. Patterns of ubiquitous β‐carotene, Chl a, and pheophytin a suggested that total algal biomass increased ∼200–400% by the mid‐1980s, but declined to near‐baseline under severe acidification. Variance partitioning using redundancy analysis captured 80–83% of variation in fossil chlorophylls and carotenoids and suggested that the direct effects of pH were greater (~50% of total variance) than those of irradiance (~12%), but that ∼20% of variance was attributable to factor interactions. Fossil concentrations of pigments from green algae and diatoms increased ~100% following acidification of Lake 302N to pH 6.1, but there were few signals of deep‐water blooms, possibly because DOC remained 3.5–5.0 mg liter−1. Such complex interactions between pH, DOC, and light may help explain the high variability of algal biomass response to lake acidification.

Effects of Phosphorus Reduction on Water Quality: Comparison of Alum-Treated and Untreated Portions of a Hypereutrophic Lake

ABSTRACT The effects of a reduction in total phosphorus concentration on the water quality and plankton community structure in a 86-ha hypereutrophic sandpit lake with high internal phosphorus loading were assessed by dosing an isolated 4.6-ha section of the lake with 34,065 L (dose = 10 mg Al·L−1) of liquid aluminum sulfate (alum). During the three summers following treatment, hypolimnetic total dissolved phosphorus, epilimnetic total phosphorus, and eplimnetic total nitrogen were decreased by 97%, 74%, and 36%, respectively, in the treated section. Secchi depth was 134% greater in the treated area. Alum treatment also increased the volume of usable fish habitat by 22%, as the depth of the 3.0-mg/l dissolved oxygen isocline was 52% deeper in the treated portion than in the untreated portion. Total phytoplankton biovolume decreased by 40% and chl a concentration by 65% in the treated area. Although cyanophytes continued to dominate in die treated area, there was a shift in relative abundance from cyanophytes to bacillariophytes and chlorophytes, especially during the second summer after treatment. By the second post-treatment summer, daphnid biomass increased substantially and there was a subtle shift in relative abundance from copepods to daphnids. Overall, alum was extremely effective in controlling sediment phosphorus release rates and lowering water column phosphorus concentrations and thus improving water clarity, reducing phytoplankton biomass, shifting phytoplankton species composition from cyanophyte dominance toward bacillariophytes and chlorophytes, increasing daphnid biomass, and increasing usable fish habitat.

Quantifying Responses to Hybrid Striped Bass Predation across Multiple Trophic Levels: Implications for Reservoir Biomanipulation

Top-down effects from piscivores to phytoplankton have been documented in north temperate lakes, thus permitting managers to regulate these food webs. From our review of the literature, reservoir trophic interactions appear less amenable to biomanipulation owing to the presence of a fast-growing, omnivorous planktivore, gizzard shad Dorosoma cepedianum. If reservoir zooplankton can be enhanced by reducing gizzard shad through biomanipulation, this could increase survival of food-limited early life stages of sport fishes dependent on zooplankton and could improve water clarity by reducing phytoplankton. To quantify this potential, we evaluated responses of age-0 gizzard shad, crustacean zooplankton, and phytoplankton to the addition of hybrid striped bass Morone saxatilis × M. chrysops (22 kg/ha) to four of eight 0.4-ha ponds. Hybrid striped bass nearly eliminated age-0 gizzard shad from ponds within 10 d after being stocked, which permitted zooplankton density and size to increase but had no effect on phytoplankton. These pond results independently confirmed the relationships among zooplankton density, zooplankton production, and age-0 gizzard shad density that had been developed previously in 1-m3 enclosures. Only reservoirs with fewer than 10 age-0 gizzard shad/m3 and daily zooplankton production greater than 220 mg/m3 may be amenable to biomanipulation; these conditions occurred in our ponds due to hybrid striped bass piscivory and high zooplankton productivity. About 5% of Ohio reservoirs possess these two characteristics, which makes the potential for improving sport fish recruitment by reducing age-0 gizzard shad abundance a limited option for reservoir managers.

SOME RECENT WATER QUALITY TRENDS IN LAKE SIMCOE, ONTARIO: IMPLICATIONS FOR BASIN PLANNING AND LIMNOLOGICAL RESEARCH

The paper presents an overview of recent limnological investigations on Lake Simcoe, with emphasis on the long-term record of dissolved oxygen, nutrients, phytoplankton, and chloride. The main sampling efforts included visits to 10–12 sites every two to three weeks during the May-October periods, as well as weekly, year-round collections of nearshore samples from four municipal water supply intakes. No major improvements in water quality in response to nutrient loading controls initiated during the last two decades were found. However, there was evidence of lower total phosphorus and phytoplankton concentrations and improved water clarity at many lake locations during the late 1980s and early 1990s. Phosphorus release from anoxic bottom sediments during late summer may retard recovery of the lake. The rate of increase of chloride in the Lake Simcoe outflow and in samples collected from municipal water supply intakes averaged 0.6 mg/L/yr over the past 20 years and likely reflects expanding urbanization in the Lake Simcoe basin. Some thoughts on future surveillance, research and planning needs are also presented.