Ecological Studies and Mathematical Modeling of Cladophora in Lake Huron: 2. Phosphorus Uptake Kinetics

Laboratory experiments were conducted with field populations of Cladophora glomerata to examine the phosphorus uptake process. Results from 35 experiments conducted over two field seasons emphasize the importance of two factors in regulating phosphorus uptake: dissolved phosphorus concentration and internal phosphorus pool size. The phosphorus uptake rate increases as dissolved phosphorus availability increases. Negative feedback on uptake rate by internal phosphorus pool size reduces the maximum phosphorus uptake rate as internal pools approach saturation. The absolute maximum phosphorus uptake rate for Cladophora was 4.5%P/day. Values for the half-saturation constant for uptake varied with internal phosphorus pool size and ranged between 30 and 250 μgP/L. The data were fit to published mathematical expressions which describe uptake as a function of internal and external phosphorus levels. The expressions may be used in mathematical models simulating the growth of Cladophora.     

Ecological Studies and Mathematical Modeling of Cladophora in Lake Huron: 4. Photosynthesis and Respiration as Functions of Light and Temperature

Laboratory experiments were conducted with Cladophora glomerata isolated from Lake Huron near Harbor Beach, Michigan, to determine gross and net photosynthesis and respiration as functions of temperature and light intensity. Rates of net photosynthesis and respiration were measured in three replicate cultures at all combinations of eight temperatures (1° to 35° C) and eight light levels (12 to 1235 μE/m²?sec) using oxygen electrode techniques. Maximum net photosynthesis occurred between 13 and 17° C and 300 to 600 μE/m²?sec. The data were converted to specific rates of growth and respiration for use in a mathematical model, and three-dimensional response surfaces were generated for gross and net growth and respiration over the entire range of light levels and temperatures examined.     

Ecological Studies and Mathematical Modeling of Cladophora in Lake Huron: 3. The Dependence of Growth Rates on Internal Phosphorus Pool Size

The relationship between growth rate and internal phosphorus pool size was examined using field populations of Cladophora glomerata from Lake Huron. Algal samples, representing a range of internal phosphorus concentrations, were harvested from the lake and used for laboratory measurements of growth. Rates of net photosynthesis and respiration were measured under controlled conditions of light and temperature using a dissolved oxygen (light/dark bottle) technique. The net specific growth rate and respiration rate were calculated from photosynthesis and respiration measurements using a fixed stoichiometric relationship and the measured carbon content of the algae. The maximum rates for net specific growth rate, gross specific growth rate, and specific respiration rate were 0.77, 1.08, and 0.44 day^?1, respectively. Management decisions may be importantly influenced by the relationships derived from these data. A non-linear response by growth to reductions in phosphorus loading is suggested from the results of these experiments. The internal nutrient status of algal populations (e.g. Cladophora) must be considered in predictions of the impact of phosphorus management strategies on aquatic systems.     

Phosphorus Uptake Dynamics as Related to Mathematical Modeling of Cladophora at a Site on Lake Huron

Cladophora is a significant symptom of eutrophication in Lakes Erie and Ontario and is a local problem associated with nutrient perturbations in Lakes Huron, Michigan, and Superior. This paper presents results of measurements of phosphorus uptake rates as a function of internal phosphorus levels by Cladophora growing near Harbor Beach, Michigan. Cladophora collected near the Harbor Beach wastewater treatment plant had high levels of internal phosphorus and low (or even negative) phosphorus uptake rates. Cladophora distant from the wastewater treatment plant had low internal phosphorus levels and rapid phosphorus uptake rates. The experimental results are discussed in terms of quantitative kinetic formulations which may be incorporated into mathematical models useful for predicting the response of Cladophora to alternative management and control strategies. Preliminary model simulations of Cladophora biomass, internal phosphorus, and external phosphorus are qualitatively similar to observed field data.     

Seasonal Abundance Patterns of Diatoms on Cladophora in Lake Huron

Rocks bearing Cladophora were collected from May to November 1979 at two locations near Harbor Beach, Michigan, in Lake Huron to document seasonal patterns of epiphytic diatom abundance and diatom proportion of the Cladophora-epiphyte assemblage biomass in an area receiving effluent from a municipal wastewater treatment plant. Data were examined for evidence of interactions between epiphytic diatoms and Cladophora.Cladophora first appeared in May at which time epiphytic diatoms comprised about 30% of the Cladophora-epiphytic assemblage biomass. Cladophora growth was greatest in June and July, accumulating much faster than diatoms. Peak Cladophora-epiphyte assemblage biomass was maintained from July to September. Cladophora biomass apparently decreased after August while diatom abundance increased to a September maximum. Diatoms were responsible for the sustained peak of Cladophora-epiphyte assemblage biomass as diatoms comprised over 60% of the assemblage biomass in September. The Cladophora-epiphyte assemblage collapsed by October.Low diversity of the epiphytic diatom taxocene and low diatom proportion of the Cladophora-epiphyte assemblage biomass indicated Cladophora may have outcompeted diatom epiphytes during the June-July Cladophora growth pulse. Subsequently epiphytic diatoms may have enhanced Cladophora sloughing by shading and successful nutrient competition.     

Residues of PCB in a Cladophora Community Along the Lake Huron Shoreline

PCB residues were measured in samples of Cladophora, Ulothrix, net plankton, water, and fish collected in the Harbor Beach area of Lake Huron. The average concentrations of total PCB in each of the sample groups were as follows: water - 0.154 ng/g; Cladophora - 92 ng/g; Ulothrix - 126 ng/g; net plankton - 1,651 ng/g; and fish - 40 to 4,400 ng/g. The PCB patterns in the samples were found to match a mixture of Aroclo6r 1242 and 1254. Aroclor 1242 represented about 60% of the mixture in all the samples except the fish, where Aroclor 1254 represented about 75% of the total PCB measured. The accumulation factors observed in the progression from Cladophora to net plankton to fish were 1:15:30; these were similar to the concentration factors (1:14:44) developed on these ecosystem compartments in Lake Ontario during the 1972 IFYGL program.     

The Great Lakes Cladophora Model: Development,testing, and application to Lake Michigan

A recent review of the Great Lakes Water Quality Agreement has concluded that while controls on phosphorus inputs to Lake Michigan achieved the desired effect in offshore waters, the nearshore region continues to suffer from elevated phosphorus levels. Failure to achieve trophic state goals in the nearshore is manifested in nuisance growth of Cladophora and attendant impacts on property owners, utilities, and the public health and welfare. This study focuses on a site in Lake Michigan near Milwaukee, Wisconsin, where nuisance growth of Cladophora and associated beach fouling occur regularly. A mechanistic model simulating Cladophora growth, suitable for guiding nutrient management in the Great Lakes nearshore, is presented. The model represents an update of the Canale and Auer framework, reflecting current understandings of Cladophora ecology and offering a user-friendly interface making the software more widely available to decision makers. This Great Lakes Cladophora Model (GLCM) is first validated for the Auer/Canale data set collected in 1979 at a site on Lake Huron and then for a data set developed in 2006 for a site on Lake Michigan. Model performance under the strikingly different forcing conditions (depth, light, phosphorus levels) characteristic of these two sites affirms the widespread applicability of the tool. The GLCM is then extended to examine the impacts of ecosystem perturbation (dreissenid colonization) on Cladophora growth and to future approaches to monitoring and management.     

Ecological Interactions Among Bangia, Cladophora, and Ulothrix Along the Lake Erie Shoreline

Bangia atropurpurea, a recent invader of Lake Erie, has altered the community structure of the littoral zone, which previously had been dominated by Cladophora or Ulothrix. Vertical and horizontal zonation patterns of these algae suggest that competition and differing physical tolerances may be involved. To determine the relative importance of these factors, seasonal changes in littoral algal distribution patterns and environmental conditions were monitored. Additionally, Bangia was examined in culture to relate growth and reproductive mechanisms to distribution patterns and competitive potential.Ulothrix established extensive cover immediately below mean water level (MWL) after thaw in mid-April along moderately exposed shores. By early May, Bangia colonized from +0.5–1.0 m to just below MWL coexisting with Ulothrix near MWL. As water temperature neared l5° C in late May, the distribution of Cladophora expanded from patches at ?0.5–1.0 m upward into the Ulothrix band. In early June, Ulothrix sporulated, the remaining empty filaments disintegrated, and the alga was not observed for the remainder of the summer. Cladophora continued upward colonization, eventually dominating the substrate to the +0.5–1.0 m level. Bangia was dominant above the Cladophora zone, up to +2.0 m. During periods of moderate wave wash, Cladophora dominated up to the +1.0 m level. Heavy wave wash resulted in the detachment of Cladophora, allowing domination by Bangia from +0.5 m to 2.0 m.Bangia shares substrate with Ulothrix in the limited areas where their distributions overlap, and perhaps through greater desiccation tolerance can persist in areas where Cladophora does not grow. This partial niche segregation provides Bangia with refugia in time as well as space. Coexistence between Bangia and the competitively dominant Cladophora occurs as a result of partial niche segregation as well as the impact of wave action in initiating the detachment of Cladophora.     

Abundance Patterns of Diatoms on Cladophora in Lake Huron with Respect to a Point Source of Wastewater Treatment Plant Effluent

Epiphytic diatoms on Cladophora were collected from July to September, 1979, at various distances from a point source of municipal wastewater treatment plant effluent to Lake Huron. Diatom accumulation rates were positively related to distance from the point source. Despite higher nutrient concentrations nearest the point source, low accumulation rates of Cladophora-epiphyte assemblage biomass and epiphytic diatoms indicated moderate growth inhibition by an undetermined factor. Evaluation of epiphytic diatom population abundance and ecology plus diatom taxocene species diversity supported our conclusions. A curvilinear relationship between diatom species diversity and impacts of growth inhibition is proposed and discussed. The diatom proportion of Cladophora-epiphyte assemblage biomass was positively related to distance from the point source of sewage effluent.     

Environmental Factors Controlling Physiological Changes in Cladophora in Lake Erie

Several physiological characteristics of Cladophora glomerata from eastern Lake Erie were monitored frequently during the summer of 1977. Soluble reactive phosphorus and nitrate-nitrogen in the lake water were also measured.Cladophora biomass generally increased throughout June, although there were periodic declines. In mid-July, biomass sharply decreased to a low level. Chlorophyll content tended to be high during periods of increasing biomass but was lower after each biomass decline. Changes in biomass did not correlate with changes in either phosphate-phosphorus or nitrate-nitrogen in the lake water. Cellular nitrogen levels fluctuated independent of lake water nitrate-nitrogen concentrations. Cellular nitrogen levels remained above the critical concentration, 1.1%-N. Luxury phosphorus levels were observed to increase following pulses of soluble orthophosphate in the lake water. Measurements of cellular total phosphorus indicate that levels remain well above the critical concentration of 0.06%-P. It is concluded that neither nitrogen nor phosphorus is limiting the growth of Cladophora at the sites sampled. The mid-summer die-off does not appear to be due to a simple nutrient deficiency.     

Modeling of Phytoplankton-Nutrient Dynamics in Saginaw Bay,Lake Huron

A multi-class, phytoplankton simulation model was developed and calibrated to an extensive set of field data acquired on Saginaw Bay, Lake Huron, during 1974. Phytoplankton biomass was partitioned into five functional groups: diatoms, greens, non-N₂-fixing blue-greens, N₂-fixing blue-greens, and “others”. Nutrients included in the model were phosphorus, nitrogen, and silicon. The model was applied to a single spatial segment encompassing the inner portion of Saginaw Bay.Process level analyses were conducted with the calibrated model to determine the relative importance of various factors affecting phytoplankton and nutrient dynamics. The concept of a single limiting factor for phytoplankton growth was found to be overly simplistic. Results indicated that temperature and light were relatively more growth rate limiting than nutrients on an annual average basis. However, as a consequence of nutrient depletion, nutrients became relatively more important at the times of peak phytoplankton crops. Nitrogen was found to be relatively more growth rate limiting than phosphorus to the total phytoplankton crop, although important differences occurred among the individual functional groups. At various times, and for various groups, all three nutrients were important in limiting either the rates of growth and/or the maximum sizes of the phytoplankton crops. Results were consistent with the hypothesis that while nitrogen and silicon were important in phytoplankton-nutrient dynamics, the supply of phosphorus would ultimately determine the size of the blue-green component of the total crop because N₂-fixing blue-greens do not have absolute requirements for dissolved available nitrogen or silicon. Results indicated that phosphorus requirements of spring and fall diatom crops were satisfied primarily by external loadings. Phosphorus requirements of summer blue-green crops were satisfied primarily by recycle processes within the water column. Upon cell death, direct nutrient recycle to the available nutrient compartments in the water column from excess internal phytoplankton stores was found to be important for both phosphorus and nitrogen. Phytoplankton production was found to be extremely sensitive to variations in the light extinction coefficient in the water column, and relatively insensitive to variations in incident solar radiation.     

Historical and recent biomass and food web relations of Limnocalanus in Lake Huron

Reported population biomass of Limnocalanus macrurus in Lake Huron has been underestimated by several-fold owing to application of an inappropriate length–weight regression model. During August and September the underestimates can exceed three-fold, suggesting that secondary production of the species in the lake is greater than previously thought. Increased representation of the species in the plankton community in recent years is associated with a decrease in its trophic level deduced through nitrogen stable isotope analysis of specimens from opportunistic samples obtained in 1993, 1995, and 2009. Limnocalanus¹⁵N enrichment relative to primary herbivores appears to have decreased by 1.5‰ over this time period. Similarly, Diaptomus¹⁵N enrichment relative to primary herbivores appeared to decrease by 1.1‰ from 1993 to 2009. Our data offer at least two possible explanations for this shift. (1) Limnocalanus has adopted a more herbivorous diet in response to the ecological changes in Lake Huron or (2) hypolimnetic Diaptomus have adopted a more herbivorous diet and Limnocalanus is relying in part on Diaptomus as a food source.     

Spatiotemporal patterns of water quality in Lake Ontario and their implications for nuisance growth of Cladophora

This study, motivated by a resurgence in Cladophora, investigates changes in the nutrient environment in the littoral zone of Lake Ontario. We measured nutrient concentrations from 2004 to 2008 at two littoral zone (2–12 m) sites on the north shore of Lake Ontario where Cladophora has experienced a resurgence and compared concentrations with data collected in the late 1970s. Spring total phosphorus (TP) and soluble reactive P (SRP) concentrations have significantly declined at these two sites. Furthermore, P loading from the major tributaries to our study sites declined between 1964 and 2008. Upwelling events were not detectably associated with increases in P concentrations at our sites. We conclude that a recent upsurge in nuisance Cladophora, at least at these sites, cannot be explained by deteriorating littoral zone water quality in terms of P concentrations or by changes in catchment loading. For additional context, we also examined trends in coastal (14–20 m) and offshore (> 50 m) nutrients using Environment Canada epilimnetic surveillance data, 1975–2008. Significant declines in TP and SRP concentrations have occurred in north coast waters, concurrent with declines in the offshore. However, nutrient concentrations, notably spring SRP, have not decreased among south coast stations, potentially reflecting greater coastal entrapment of catchment-derived waters. We infer that EC-monitored north coast stations reflect integrated interannual water quality, while south coast stations are more strongly influenced by catchment loading. The effects of higher nutrient concentrations along the south coast, which co-occur with lower water transparency, on benthic algal growth have yet to be determined.     

Progress Toward the Rehabilitation of Lake Trout (Salvelinus namaycush) in South Bay,Lake Huron

Our objective was to evaluate the status of lake trout Salvelinus namaycush rehabilitation in South Bay, Lake Huron. Standardized surveys were conducted to quantify natural recruitment, annual mortality, and the contribution of wild-versus hatchery-origin lake trout. Some indicators suggest a high level of natural recruitment. The spawning population was comprised of multiple ages, and the mean age of spawners (8.4 years for females, 7.9 years for males) was at least 1 year older than the age at 50% maturity (5.8 years). Estimated annual total mortality rates (0.20–0.25) and sea-lamprey induced mortality rates (0.02) were less than maximum allowable values. The proportion of wild-origin fish captured was high among spawners but varied among sampling programs (42% in fall trap nets, 70% in fall gill nets, and 88% in summer gill nets). A strong year class (1997) could be tracked from 2001 to 2005. Few fish were captured from early (< 1996) or later (1999–2002) year classes. Possible explanations for low natural recruitment during these later years include declining spawning habitat quality caused by low water levels and/or invasion of non-native mussels (Dreissena spp.) and/or direct or indirect effects of alewife (Alosa pseudoharengus).     

Fall diet and bathymetric distribution of deepwater sculpin (Myoxocephalus thompsonii) in Lake Huron

Deepwater sculpin Myoxocephalus thompsonii are an important component of Great Lake's offshore benthic food webs. Recent declines in deepwater sculpin abundance and changes in bathymetric distribution may be associated with changes in the deepwater food web of Lake Huron, particularly, decreased abundance of benthic invertebrates such as Diporeia. To assess how deepwater sculpins have responded to recent changes, we examined a fifteen-year time series of spatial and temporal patterns in abundance as well as the diets of fish collected in bottom trawls during fall of 2003, 2004, and 2005. During 1992–2007, deepwater sculpin abundance declined on a lake-wide scale but the decline in abundance at shallower depths and in the southern portion of Lake Huron was more pronounced. Of the 534 fish examined for diet analysis, 97% had food in the stomach. Mysis, Diporeia, and Chironomidae were consumed frequently, while sphaerid clams, ostracods, fish eggs, and small fish were found in only low numbers. We found an inverse relationship between prevalence of Mysis and Diporeia in diets that reflected geographic and temporal trends in abundance of these invertebrates in Lake Huron. Because deepwater sculpins are an important trophic link in offshore benthic food webs, declines in population abundance and changes in distribution may cascade throughout the food web and impede fish community restoration goals.     

Mass Balance Modeling of Heavy Metals in Saginaw Bay,Lake Huron

During the period 1976-1978, a study of hazardous materials in Saginaw Bay was conducted. This study included the fate and distribution of cadmium, copper, lead, and zinc in the bay. A spatially segmented, dynamic mass balance model was developed to describe concentrations of metals and suspended solids in the water column and in the sediments. A wind-driven resuspension mechanism was used to describe the sediment-water interactions. The distribution of metals in the water column was determined by equilibrium partitioning between the ambient suspended solids and the dissolved phase. Model output was calibrated to field data for the principal variables. Independent validation was obtained by comparing partition coefficients from the calibration to those calculated directly from the field observations. It was found that suspended solids were important in controlling the water column concentrations of the metals. The degree of control was a function of the partition coefficient between the metal and the solids, and the concentration of the solids. Adsorption of the metals to the solids was found to result in decreases to metals concentrations due to net sedimentation, as well as increases due to wind-driven resuspension. On an annual average basis, the net flux of the particulate components of all four metals was from the water column to the sediments except for copper in 1977.