Stephanie J. Guildford

The Light: Nutrient Ratio in Lakes: The Balance of Energy and Materials Affects Ecosystem Structure and Process

The amounts of solar energy and materials are two of the chief factors determining ecosystem structure and process. Here, we examine the relative balance of light and phosphorus in a set of freshwater pelagic ecosystems. We calculated a ratio of light: phosphorus by putting mixed‐layer mean light in the numerator and total P concentration in the denominator. This light: phosphorus ratio was a good predictor of the C:P ratio of particulate matter (seston), with a positive correlation demonstrated between these two ratios. We argue that the balance between light and nutrients controls “nutrient use efficiency” at the base of the food web in lakes. Thus, when light energy is high relative to nutrient availability, the base of the food web is carbon rich and phosphorus poor. In the opposite case, where light is relatively less available compared to nutrients, the base of the food web is relatively P rich. The significance of this relationship lies in the fact that the composition of sestonic material is known to influence a large number of ecosystem processes such as secondary production, nutrient cycling, and (we hypothesize) the relative strength of microbial versus grazing processes. Using the central result of increased C:P ratio with an increased light: phosphorus ratio, we make specific predictions of how ecosystem structure and process should vary with light and nutrient balance. Among these predictions, we suggest that lake ecosystems with low light: phosphorus ratios should have several trophic levels simultaneously carbon or energy limited, while ecosystems with high light: phosphorus ratios should have several trophic levels simultaneously limited by phosphorus. Our results provide an alternative perspective to the question of what determines nutrient use efficiency in ecosystems.

AN ECOLOGICAL REVIEW OF CLADOPHORA GLOMERATA (CHLOROPHYTA) IN THE LAURENTIAN GREAT LAKES1

Cladophora glomerata (L.) Kütz. is, potentially, the most widely distributed macroalga throughout the world’s freshwater ecosystems. C. glomerata has been described throughout North America, Europe, the Atlantic Islands, the Caribbean Islands, Asia, Africa, Australia and New Zealand, and the Pacific Islands. Cladophora blooms were a common feature of the lower North American Great Lakes (Erie, Michigan, Ontario) from the 1950s through the early 1980s and were largely eradicated through the implementation of a multibillion‐dollar phosphorus (P) abatement program. The return of widespread blooms in these lakes since the mid‐1990s, however, was not associated with increases in P loading. Instead, current evidence indicates that the resurgence in blooms was directly related to ecosystem level changes in substratum availability, water clarity, and P recycling associated with the establishment of dense colonies of invasive dreissenid mussels. These results support the hypothesis that dreissenid mussel invasions may induce dramatic shifts in energy and nutrient flow from pelagic zones to the benthic zone.

The Wall of Green: The Status of Cladophora glomerata on the Northern Shores of Lake Erie's Eastern Basin, 1995–2002

The biomass, areal coverage, algal bed characteristics, and tissue phosphorus concentrations of Cladophora glomerata were measured at 24 near shore rocky sites along the northern shoreline of Lake Eries eastern basin between 1995–2002. Midsummer areal coverage at shallow depths (≤ 5 m) ranged from 4–100%, with a median value of 96%. Cladophora biomass began accumulating at most sites during early May, and achieved maximum values by mid-July. Peak seasonal biomass ranged from < 1 to 940 g/m2 dry mass (DM), with a median value of 171 g/m2 DM. Nearshore water concentrations of total phosphorus (TP) were lower than during pre-phosphorus abatement years. However, Cladophora biomass levels were similar to reported values in those years. The midsummer “die off” occurred shortly after the biomass peak, when water temperatures neared 22.5°C. Areal coverage declined after die-off to < 10%, mean filament lengths declined from 33 cm to < 1 cm, and mean biomass declined to < 1 g DM/m2. Tissue phosphorus varied seasonally, with initial high values in early May (0.15 to 0.27% DM; median 0.23% DM) to midsummer seasonal low values during peak biomass (0.03 to 0.23% DM; median 0.06% DM). Cladophora biomass is sensitive to changes in phosphorus and light availability, and reductions in biomass previously achieved through phosphorus control may now be reversed because of increased water transparency and phosphorus availability to the benthos following establishment of dreissenids.

Evaluating Microcystin Exposure Risk through Fish Consumption

Microcystin is a cyanobacterial hepatotoxin that is found worldwide, and poses a serious threat to the ecological communities in which it is found as well as to those who rely on these waters for drinking, sanitation, or as a food source. Microcystin is known to accumulate in fish and other aquatic biota, however the prevalence of microcystin in fish tissue and the human health risks posed by microcystin exposure through fish consumption remain poorly resolved. Here we show that microcystin is pervasive in water and fish from several tropical (Ugandan) and temperate (North American) lakes, including lakes that support some of the largest freshwater fisheries in the world. We establish that fish consumption can be an important and sometimes dominant route of microcystin exposure for humans, and can cause consumers to exceed recommended total daily intake guidelines for microcystin. These results highlight the importance of monitoring microcystin concentrations in fish, and the need to consider potential exposure to microcystin through fish consumption in order to adequately assess human exposure risk.

Nutrient Enrichment Experiments in Tropical Great Lakes Malawi/Nyasa and Victoria

Enrichment experiments with and without zooplankton (> 50 μm) removed were conducted in Lake Malawi during three seasons (stratified rainy, deep mixing, and stratified dry) and demonstrated that when light is adequate phytoplankton in containers quickly become nutrient deficient. The response to enrichment was assessed using chlorophyll a, photosynthesis, particulate stoichiometric ratios, PO4 turnover and N and P debt assays. The response to nitrogen (N), phosphorus (P), and iron (Fe) enrichments indicated that although N is the nutrient that becomes deficient most consistently, P deficiency is common as well. When Fe was added with N and P, the response by chlorophyll was four times the response to N and P without Fe. This suggests that, after N and P, Fe is the next most limiting nutrient in Lake Malawi. Light was a factor controlling phytoplankton growth in situ during the deep mixing season, and grazer removal experiments demonstrated that zooplankton > 50 μm are important in modifying the response of algae to light and nutrients. In Lake Victoria, experiments demonstrated that phytoplankton were primarily light-limited during the early-stratified season. Increased light levels resulted in N deficiency. Fe additions stimulated N uptake in both Lake Victoria and Lake Malawi and N2 fixation in Lake Victoria.

Phytoplankton Nutrient Status in Lake Erie in 1997

Abstract Reduction of phosphorus (P) inputs to Lake Erie since the 1970s have resulted in lower mean total phosphorus (TP) concentration in the lake and lower mean chlorophyll a concentration. In this study we examine indicators of phytoplankton nutritional status to determine whether the observed decrease in TP concentration has resulted in a strongly P deficient phytoplankton community. Phytoplankton nutrient status measurements including alkaline phosphatase activity (APA), P and nitrogen (N) debt, PO 4 turnover time and particulate carbon (C), N and P stoichiometric ratios, were made in all three basins of Lake Erie from May through October in 1997. The phytoplankton in the western basin only infrequently showed signs of nutrient deficiency over the May through October sampling season. Phytoplankton in the large central basin were moderately P deficient during the stratified season except for a period of strong deficiency in July, the time of maximum heat content and strong stratification. The eastern basin became moderately P deficient with the onset of stratification and remained moderately P deficient over the entire stratified season and no indications of extreme deficiency were observed. Compared to several other lakes in the same geoclimatic region as Lake Erie, including Lake Superior, the phytoplankton community was not strongly nutrient deficient.

Phytoplankton nutrient status and mean water column irradiance in Lakes Malawi and Superior

Abstract Phytoplankton growth in Lake Malawi was moderately nitrogen and phosphorus-deficient according to indicators of phytoplankton nutrient status (particulate C:N, C:P, N:P composition ratios, nitrogen and phosphorus debt assays) and occasionally light-deficient during the period of deepest mixing (July and August). Phytoplankton in Lake Superior was light-deficient during most of the year because of the deeply mixed water column. However, during the stratified period when the mean water column irradiance increased, phytoplankton in Lake Superior became severely phosphorus-deficient according to the same nutrient status indicators used in Lake Malawi as well as alkaline phosphatase activity. Specific rates of carbon uptake normalized to particulate carbon, calculated from photosynthesis at optimum light, were on average three times greater in Lake Malawi than in Lake Superior. We calculated that nitrogen and phosphorus inputs from rivers and precipitation supplied < 15% of the demand for these elemen...

Evaluation of the human health threat associated with the hepatotoxin microcystin in the muscle and liver tissues of yellow perch (Perca flavescens)

During the summer of 2006, the western basin of Lake Erie experienced a bloom of the toxigenic cyanobacte- rium Microcystis aeruginosa. Across 11 sites, intracellular, particulate-bound microcystin levels in the seston increased to levels that exceeded World Health Organization guidelines for drinking water exposure (1 mg toxinL -1 ). In contrast, toxin concentrations in yellow perch (Perca flavescens) muscle tissue (n = 68) declined from June to August, were negatively related to algal toxin levels, and never exceeded a conservative chronic exposure concentration estimated using proposed United States Environmental Protection Agency (US EPA) guidelines. Microcystin concentrations in yellow perch liver exceeded US EPA chronic exposure guidelines, were on average 125 times higher than muscle toxin concentrations per unit dry weight, and varied little throughout the summer. With current guidelines, humans do not appear to be at risk when consuming the muscle tissue of Lake Erie yellow perch collected during large-scale cyanobacterial blooms. How- ever, this study highlights the need for a better understanding of the trophic transfer of cyanobacterial toxins through aquatic food webs in diverse ecosystems with an emphasis on understanding if these compounds could accumulate suffi- ciently to affect human health.

Environmental Controls of Cladophora Growth Dynamics in Eastern Lake Erie: Application of the Cladophora Growth Model (CGM)

ABSTRACT The Cladophora growth model (CGM) was used to estimate the importance of light, temperature, phosphorus, and self-shading on the spatial and temporal variability of Cladophora growth rates and biomass accrual in eastern Lake Erie during 2002. The CGM predicted that Cladophora growth was highly sensitive to spatial and temporal variations in soluble phosphorous concentration (SRP). Specifically the CGM predicted that: 1) Site-to-site differences in SRP concentration resulted in a 2×difference in depth-integrated biomass; 2) maximum growth rates were strongly influenced by SRP concentrations during periods of rapid biomass accrual (mid-June to mid-July); 3) inter-annual differences in SRP concentration during the spring period (∼ 1 μg/L) could result in up to a 3.5×difference in depth integrated biomass; 4) Spatial variations in water clarity could result in a 2×difference in depth-integrated biomass between sites, with variations betweens sites occurring primarily between 2–6 m depth; 5) the midsummer sloughing phenomenon likely resulted from self-shading by the algal canopy; and 6) the seasonal growth pattern of Cladophora was strongly regulated by temperature.

Modeling the Growth, Biomass, and Tissue Phosphorus Concentration of Cladophora glomerata in Eastern Lake Erie: Model Description and Field Testing

Cladophora glomerata is a filamentous alga that currently forms extensive blooms in nearshore areas of Lake Ontario, eastern Lake Erie, Lake Michigan, and isolated locations in Lake Huron. During the late 1970s an extensive effort was put forward to model Cladophora growth and biomass accrual based on several highly dynamic ecological variables including: Photosynthetically active radiation (PAR), soluble reactive phosphorus (SRP), water temperature, and carrying capacity (Canale and Auer 1982a). The original “Canale and Auer” model was developed and validated in proximity to a sewage treatment outfall in Lake Huron and predicted Cladophora growth and biomass over a range of SRP concentrations, at shallow depths (0–3 m), with reasonable accuracy. We present a revised version of the “Canale and Auer” model, which we refer to as the Cladophora Growth Model (CGM). The CGM expands its utility of the Canale and Auer model to greater depths and to areas of non-point source P loading while reducing the quantity of input data required. The CGM was incorporated into a computer simulation model using Stella modeling software, and is available from the corresponding author of this manuscript. The CGM was tested over a single growing season at five sites, and three depths (2, 5, 10 m), that represented a wide geographical distribution and expected range in ecological conditions in eastern Lake Erie. The CGM predicted growth, biomass, and tissue phosphorus concentrations with reasonable accuracy. The revised model is useful for: 1) Predicting Cladophora growth, biomass, and tissue phosphorus concentrations under non-point source P loading with no depth restrictions; 2) providing estimates of the timing and magnitude of the midsummer sloughing phenomenon; 3) determining the contribution of Dreissena invasion to the resurgence of Cladophora in eastern Lake Erie; and 4) developing management strategies for Cladophora abatement.