Joseph D. Conroy

Temporal Trends in Lake Erie Plankton Biomass: Roles of External Phosphorus Loading and Dreissenid Mussels

Abstract We compare the results of lakewide plankton studies conducted during 1996–2002 with data reported in the literature from previous years to evaluate the effectiveness of continued nutrient control, the relationship between external phosphorus loading and plankton abundance, and the many predicted outcomes of the dreissenid invasion. We found that although recent external annual phosphorus loading has not changed since reaching mandated target levels in the early- to mid-1980s, phytoplankton communities have. Total phytoplankton biomass, measured through enumeration and size-frequency distributions has increased since minima were observed in 1996 or 1997, with summer (July–September) biomasses generally greater than before the dreissenid establishment in the late 1980s. Cyanobacteria biomass also increased during summer in all basins after the dreissenid invasion. In contrast, chlorophyll a concentration has decreased in all basins during both spring and summer. However chlorophyll a concentration was poorly correlated with total phytoplankton biomass. Relative to the mid-1980s, crustacean zooplankton biomass during the years 1996–2002 increased in the western basin during spring and summer, increased in the central basin during spring but remained the same during summer, and decreased to low levels in the eastern basin. Several of these observations are consistent with predictions made by previous researchers on the effects of reduced total external phosphorus loading and the stimulatory or inhibitory effects of dreissenid mussels. However, several were not. Results from this study, particularly the inconsistencies with tested predictions, highlight the need for further research into the factors that regulate plankton community dynamics in Lake Erie.

Do Dreissenid Mussels Affect Lake Erie Ecosystem Stability Processes

Abstract Ecosystem stability processes such as constancy, resilience and persistence are important, but often neglected, topics of invasive species research. Here we consider how invasive dreissenid mussels affect ecosystem stability processes in Lake Erie through both consumptive and excretory processes using the stability landscape heuristic (Gunderson, 2000). Consumption of phytoplankton by dreissenid mussels adds complexity to the system and potentially slows energy transfer from lower to higher trophic levels decreasing system constancy and lowering system resiliency. Excreting soluble waste products at low nitrogen to phosphorus ratios exacerbates these impacts on stability processes because low nutrient ratios favor growth of cyanobacterial blooms, less preferred food of zooplankton, further decreasing the transfer of energy from lower to higher trophic levels. We also provide evidence for recent changes in Lake Eries stability landscape including a return towards eutrophy.

Cylindrospermopsis in Lake Erie: Testing its Association with Other Cyanobacterial Genera and Major Limnological Parameters

ABSTRACT We report the first documented observation of the potentially toxic cyanobacterium Cylindrospermopsis in Lake Erie and Sandusky Bay in 2005 (0.043–1.326 mg L−1 wet weight, 16–1,942 trichomes mL−1) and quantify the physical and chemical parameters and the cyanobacterial community composition contemporaneous to its occurrence. We hypothesize that the high temperature, low light intensity, and high nutrient content of Sandusky Bay, a shallow, drowned river mouth along the southwestern shore of Lake Erie, provides an ideal habitat for Cylindrospermopsis. This is consistent with published laboratory and field studies that show these physical and chemical conditions facilitate Cylindrospermopsis growth. Using multivariate statistics, we found that Cylindrospermopsis biomass correlated with high temperatures and shallow depths, conditions often found in Sandusky Bay. Light climate and nutrient concentrations were not associated with Cylindrospermopsis biomass, most likely because the light climate did not systematically change during the season and because nutrients exceeded demand. We propose that Cylindrospermopsis will increase in importance in Lake Erie, as previous research on climate change in the Great Lakes region predicts future higher water temperatures and lower water levels.

Hypolimnetic Oxygen Depletion Dynamics in the Central Basin of Lake Erie

Hypolimnetic oxygen depletion has been recognized as a problem in the central basin of Lake Erie since the 1970s. However, recent expansion in distribution of the depletion after several years of low depletion rates in the 1990s has led investigators to explore the factors that influence the extent of the depletion. We have investigated the vertical oxygen budget in the central basin, which is influenced by the following factors: 1) vertical mixing; 2) exchange across the air-water interface; 3) photosynthesis; 4) respiration of plankton; and 5) sediment oxygen demand. We tested the importance of these factors using a 1-D vertical oxygen budget and transport simulations through sensitivity analysis and by estimating vertical mixing parameters using a temperature gradient microprofiler. Epilimnetic factors were found to be robust and the present monitoring efforts are sufficient; while epilimnetic production is ultimately the source of the hypolimnetic oxygen depletion, epilimnetic factors do not directly influence on hypolimnetic oxygen depletion. However, hypolimnetic depletion was sensitive to sediment oxygen demand and hypolimnion respiration, which are the results of primary production in the epilimnion, and hypolimnetic mixing, which is not related to eutrophication. These parameters, especially the physical mixing measurements, and their links with eutrophication and primary production require greater monitoring and analysis because of their influence on the expansion of oxygen depletion in the central basin of Lake Erie.

Transient Social–Ecological Stability: the Effects of Invasive Species and Ecosystem Restoration on Nutrient Management Compromise in Lake Erie

Together, lake ecosystems and local human activity form complex social-ecological systems (SESs) characterized by feedback loops and discontinuous change. Researchers in diverse fields have suggested that complex systems do not have single stable equilibria in the long term because of inevitable perturbation. During this study, we sought to address the general question of whether or not stable social- ecological equilibria exist in highly stressed and managed lacustrine systems. Using an integrated human- biophysical model, we investigated the impacts of a species invasion and ecosystem restoration on SES equilibrium, defined here as a compromise in phosphorus management among opposing stakeholders, in western Lake Erie. Our integrated model is composed of a calibrated ecological submodel representing Sandusky Bay, and a phosphorus management submodel that reflects the societal benefits and costs of phosphorus regulation. These two submodels together form a dynamic feedback loop that includes freshwater ecology, ecosystem services, and phosphorus management. We found that the invasion of dreissenid mussels decreased ecosystem resistance to eutrophication, necessitating increased phosphorus management to preserve ecosystem services and thus creating the potential for a shift in social-ecological equilibrium. Additionally, our results suggest that net benefits in the region following the invasion of dreissenids may never again reach the pre-invasion level if on-site phosphorus control is the sole management lever. Further demonstrating transient system stability, large-scale wetland restoration shifted points of management compromise to states characterized by less on-site phosphorus management and higher environmental quality, resulting in a significant increase in net benefits in the region. We conclude that lacustrine SESs are open and dynamic, and we recommend that future models of these systems emphasize site-specific perturbation over equilibrium, thereby aiding the development of management plans for building system resistance to undesirable change that are both flexible and sustainable in an unknowable future.

The importance of phosphorus and copper carryover in pond sediments for juvenile percid survival, growth, and yield.

Abstract Over-fertilization with phosphorus and carryover copper residual in the sediment from copper sulfate treatment in double-cropped ponds were suspected as the primary causes of low larval percid survival in coolwater aquaculture ponds. Consequently, in this study, we compared saugeye (male sauger Sander canadensis × female walleye S. vitreus) survival, growth, and yield among ponds that were single-cropped with only saugeyes in the spring during the previous two consecutive years (SS ponds) and ponds that were double-cropped (with saugeye culture in the spring and channel catfish Ictalurus punctatus culture in the summer) during the previous two consecutive years (DD ponds). We also compared sediment phosphorus (soluble reactive phosphorus [SRP] and total phosphorus [TP]), sediment copper concentrations, phytoplankton biomass, and zooplankton biomass. Fish survival (number harvested) and fish yield (mass harvested) were significantly lower in DD ponds than in SS ponds in 2001 but were similar in 20...

Gloom and blooms: simulating phytoplankton growth moving out of a tributary into a large lake

Most studies of large lake trophic dynamics simply consider within-lake processes. For example, the classic eutrophication models modeled lower trophic level ( e.g., chlorophyll a concentration [ehl-a]) relationships to nutrient dynamics (e.g., total phosphorus concentrations) as direct linear regressions or as linear regressions with time lags (OEeD 1982). If tributaries were considered at all, they were modeled only as nutrient sources (OEeD 1982) and lacked a biological load factor. Treating tributaries solely as nutrient sources is not restricted to studies performed decades ago. Even recent, well-cited studies restrict tributaries to serving as aseptic nutrient inputs to a lacustrine chemostat (see Fig. l in eARPENTER et al. 1998), even though tributaries contain particulate matter including phytoplankton (MALMAEUS & HAKANSON 2003). eoastal marine studies recognize the important role of tributaries as particle sources and emphasize their study to better understand offshore processes (RABALAIS et al. 1996, PAERL et al. 1998). Recognizing that the health of Lake Eri e (USA, eanada) has declined in recent years (e.g., post-1995) as evidenced by increases in total phytoplankton biomass (eoNROY et al. 2005a), cyanobacteria biomass (eoNROY et al. 2008), and recurrent areas ofhypolimnetic hypoxia and anoxia (BuRNS et al. 2005), we sought to test whether (l) external phosphorus loading increased; (2) invasive dreissenid mussels altered Lake Eries nutrient budget by increasing interna! nutrient remineralization (recycling); and/or (3) tributary-discharged phytoplankton formed a critical inoculum that increased the offshore phytoplankton growth rate, increasing total organic matter burden to the offshore. We examined the first 2 hypotheses previously (eoNROY et al. 2005a, 2008 and eoNROY et al. 2005b, respectively) and tested the third hypothesis in this study and in a recent dissertation that examined the Algal Loading Hypothesis (eoNROY 2007). Here we specifically examine the sensitivity of loaded tributary phytoplankton growth rate to changes in the light climate (as the attenuation coefficient, k) and where it is loaded to (e.g., offshore epilimnion, offshore hypolimnion, nearshore), hypothesizing that loaded phytoplankton can affect greatly both nearshore and offshore phytoplankton productivity.

Abiotic and biotic controls of phytoplankton biomass dynamics in a freshwater tributary, estuary, and large lake ecosystem: Sandusky Bay (Lake Erie) chemostat

Abstract The classic view of tributary–lake interactions solely considers tributaries as sources of nutrients for phytoplankton growing offshore. We tested how river input of phytoplankton affected dynamics in a lacustrine estuary and how these phytoplankters could, upon discharge, affect offshore phytoplankton dynamics. In addition, we explored whether phytoplankton biomass was controlled by bottom-up abiotic factors (stream flow, temperature, light attenuation, and nutrient concentrations) or top-down biotic factors (zooplankton herbivory) and the role the lacustrine estuary played in modifying the quality of the water discharged into the lake. We tested these possibilities using field sampling over 2 years in the Sandusky River, Sandusky Bay, and the Sandusky subbasin of Lake Erie in conjunction with river flow and nutrient monitoring and multivariate statistical analysis of the relationships among the various datasets. Nutrient loading to the estuary corresponded with watershed input; nutrients were used within the estuary, decreasing nutrient loads to the lake. Phytoplankton biomass, however, was extremely high in the estuary and was much higher at the bay–subbasin interface than offshore, indicating that the estuary likely discharges phytoplankton to the nearshore area, not offshore. An information-theoretic analysis of 15 candidate models showed that phytoplankton biomass was best predicted by the ratio of total inorganic nitrogen to total phosphorus, indicating bottom-up control. The lacustrine estuary therefore functions as a chemostat, using incoming nutrients to grow large amounts of phytoplankton. Our results indicate that the classic model of tributary–bay interactions needs revision to include phytoplankton load and its subsequent effects within the lake.

“Optimal” P loading in large lakes affects fish communities: Do you prefer walleye or yellow perch?

In the Laurentian Great Lakes, recruitment of yellow perch (Percajlavescens) and walleye (Sander vitreus) is high1y variab1e. Suggested factors to exp1ain this resu1t include spawning stock size, zoop1ankton availability to juveni1es (DETIMERS et al. 2003, HoXMEIER et al. 2004, FULFORD et al. 2006), predation (HARTMAN & MARGRAF 1993), spring warming rate (BuscH et al. 1975, CLADY 1976), and temperature (HO KANsoN 1977, HENDERSON & NEPZY 1988). Recruitment of other taxa found in the smallest ofthe Laurentian Great Lakes, Lake Eri e, a1so varies, inc1uding species such as, white bass (Morone chrysops), gizzard shad (Dorosoma cepedianum), rainbow sme1t ( Osmerus mordax), andemera1d (Notropis atherinoides) and spottai1 shiners (N. hudsonius; ODW 2006). The abundances of walleye and yellow perch have increased significantly since 1970 as phosphorus (P) 1oading to the 1ake has decreased. Because P 1oading and young-of-the-year (YOY) fish abundances have been measured since 1969, Lake Eri e allows us to test whether P-1oading affects fish recruitment.