Diatoms abound in ice-covered Lake Erie: An investigation of offshore winter limnology in Lake Erie over the period 2007 to 2010

The limnology of offshore Lake Erie during periods of extensive (> 70%) ice cover was examined from ship borne sampling efforts in 2007 to 2010, inclusive. Dense and discrete accumulations of the centric filamentous diatom Aulacoseria islandica (> 10 μg Chl-a/L) were located in the isothermal (< 1 °C) water column directly below the ice and only detectable in the ship wake; viable phytoplankton were also observed within ice. Evidence from these surveys supports the notions that winter blooms of diatoms occur annually prior to the onset of ice cover and that the phytoplankton from these blooms are maintained in the surface waters of Lake Erie and reduce silicate concentrations in the lake prior to spring. The mechanisms by which high phytoplankton biomass rise at this time of year requires further investigation, but these winter blooms probably have consequences for summer hypoxia and how the lake responds to climate change.     

Oxygen use by nitrification in the hypolimnion and sediments of Lake Erie

Nitrification is an oxygen consumptive process, consuming 2 mol of oxygen per mol of ammonium oxidized. Hypolimnion and sediment samples were collected during the summers of 2008–2010 in Lake Erie to determine the total oxygen consumption and oxygen consumption from nitrification by blocking nitrification with selective inhibitors. Oxygen consumption by nitrification in the hypolimnion was 3.7 ± 2.9 (mean ± 1 SD) μmol O₂/L/d, with nitrification accounting for 32.6 ± 22.1% of the total oxygen consumption. Nitrification in the hypolimnion contributed more to oxygen consumption in the eastern sites than western sites and was lowest in September. The nitrification rate did not correlate with environmental factors such as oxygen, nitrate or ammonium, or nitrifier numbers. Oxygen consumption by nitrification in sediment slurries was 7.1 ± 5.8 μmol O₂/g/d, with nitrification accounting for 27.0 ± 19.2% of the total oxygen consumption with the lowest rates in July and the lowest percentages in June. Oxygen consumption by nitrification in intact sediment cores was 682 ± 61.1 μmol O₂/m/d with nitrification accounting for 30.4 ± 10.7% of the total oxygen consumption. Nitrification rates in intact cores were generally highest in September. The proportion of oxygen consumed by nitrification corresponds closely with what would be predicted from complete oxidation of a Redfield molecule (23%). While nitrification is unlikely to be the dominant oxygen consumptive process, the rates observed in Lake Erie were sufficient to theoretically deplete a large portion of the hypolimnetic oxygen pool during the stratified period.     

Influence of increasing populations of Double-crested Cormorants on soil nutrient characteristics of nesting islands in western Lake Erie

Animals can influence the structure of an ecosystem by changing the levels of nutrient input. This is of particular importance for the islands of western Lake Erie, which are relatively nutrient poor, but have experienced increases in nutrient input from growing double-crested cormorant (Phalacrocorax auritus) populations. The objectives of this study were to evaluate changes in soil characteristics (nutrients [nitrate (NO₃), total P], pH, and δ¹³C [as a tracer of cormorant-associated nutrients]) across a gradient of cormorant nest density on two islands (Middle and East Sister) in western Lake Erie. For both islands, soil pH decreased and P concentrations increased with nest density. On Middle Island, soil nitrate concentrations increased with cormorant nest density, and varied with breeding phenology, with highest concentrations during the early and mid nesting season (272 ± 19 μg g^− 1) and lowest concentrations late in the season (165 ± 11 μg g^− 1). Following a 3-year absence of nesting activity at sites on Middle Island, soil nitrate concentrations were similar to those at low density sites. In contrast, nitrate concentrations measured on East Sister Island did not correlate with temporal or spatial patterns of cormorant nesting and remained elevated 10 years post-cormorant use. While the results of this study confirm that chronic input of allochthonous materials alters soil properties of these islands, the unique conditions of each island must be considered when predicting ecological effects and setting long-term management objectives.     

Sediment Oxygen Demand in the Central Basin of Lake Erie

Three separate procedures were used to estimate the sediment oxygen demand (SOD) in the central basin of Lake Erie and were compared with other estimates determined previously and with historical data. First, whole core incubations involved sealing sediment cores at 12°C to ensure no interaction between the overlying water and the atmosphere and monitoring continuously to define the linear disappearance of oxygen. Second, sediment plugs were placed inside flow-through reactors and the influent and effluent concentrations were monitored to obtain steady-state reaction rates. Third, an extensive data set for the central basin of Lake Erie was compiled for input into the diagenetic BRNS model, and the SOD was calculated assuming all primary redox reactions, but no secondary reactions. All three procedures produced estimates of SOD that were in reasonable agreement with each other. Whole core incubations yield an average SOD of 7.40 × 10⁻¹² moles/cm²/sec, the flow-through experiments had an average SOD of 4.04 × 10⁻¹² moles/cm²/sec, and the BRNS model predicts an SOD of 7.87 × 10⁻¹² moles/cm²/sec over the top 10 cm of sediment and appears to be calibrated reasonably well to the conditions of the central basin of Lake Erie. These values compare reasonably well with the 8.29 × 10⁻¹² moles/cm²/sec obtained from diffusion modeling of oxygen profiles (Matisoff and Neeson 2005). In contrast, values reported from the 1960s to 1980s ranged from 10.5–32.1 × 10⁻¹² moles/cm²/sec suggesting that the SOD of the central basin has decreased over the last 35 years, presumably, in response to the decrease in phosphorus loadings to Lake Erie. However, since hypoxia in the hypolimnion persists these results suggest that improvement in hypolimnetic oxygen concentrations may lag decreases in loadings or that the hypolimnion in the central basin of Lake Erie is simply too thin to avoid summer hypoxia during most years.     

High Variability of Phytoplankton Photosynthesis in Response to Environmental Forcing in Oligotrophic Lake Malawi/Nyasa

In southern Lake Malawi, seasonal pelagic chlorophyll means were 1.0 ± 0.3 μg L⁻¹ in the deep mixing season (DMS) (May–August), 0.8 ± 0.3 μg L⁻¹ in the dry stratified season (DSS) (September to November) and 0.7 ± 0.3 μg L⁻¹ in the wet stratified season (WSS) (December to April). Despite the low variability in chlorophyll, there was a wide range in chlorophyll specific photosynthetic activity. The photosynthetic parameters, P^b_m (the light saturated rate) and α^b (the light limited slope), varied significantly among seasons and were highly positively correlated, with lowest values in the DSS and highest values in WSS. During deep mixing, P^b_m did not covary with α^b; and the light saturation index, E_k (=P^b_m/α^b), varied in response to changes in α^b rather than in P^b_m. Phytoplankton appeared to be nutrient deficient at all times but less deficient during deep vertical mixing in the DMS. Average daily rates of integrated phytoplankton primary productivity were lowest in the DSS (337 mg C m⁻² d⁻¹) and highest in the WSS (629 mg C m⁻² d⁻¹) despite nearly identical mean chlorophyll concentrations. Along a near shore transect off the Linthipe River, chlorophyll concentrations were higher and more variable (1.4 ± 1.3 μg L⁻¹), phytoplankton were not strongly nutrient deficient and chlorophyll specific photosynthetic activity was as high or higher than at the offshore station. Estimates of phytoplankton productivity in this tropical great lake must account for spatial and temporal variability in photosynthetic parameters imposed by seasonal changes in mixing dynamics.     

Algal pigments in Lake Erie dreissenids,pseudofeces and sediments,as tracers of diet,selective feeding and bioaccumulation

Algal class biomarkers revealed the importance of diatoms and chlorophytes in the diet of dreissenids (n 18) collected from eastern Lake Erie (2–20 m depths; hard and soft substrate), during summer and fall, from 2003 to 2005. The most prominent biomarkers in dreissenids, typical of August, were fatty acid esters of carotenoids derived from diatoms (monoesters of fucoxanthinol, average 61%) and chlorophytes (diesters of mactraxanthin, average 30%), while non-esterified biomarkers from diatoms, chlorophytes, cryptophytes and cyanobacteria were below 4% of the total. At cool temperatures (June 2003, 13 °C), dreissenids had 84%:12% diatom:chlorophyte biomarkers, but with unseasonably warm temperatures (June 2004, 18 °C) and a nearshore chlorophyte bloom, dreissenids had a biomarker distribution similar to August. Bioconcentration factors in dreissenids relative to phytoplankton from Lake Erie were largest for the biomarkers from diatoms (21 L/g ww) and chlorophytes (29 L/g ww), compared to those from cryptophytes (2 L/g ww) and cyanobacteria (3 L/g ww). Unlike dreissenids (2003), matching pseudofeces (n 6) and sediments (n 16) contained a relatively large percentage of biomarkers for cryptophytes (20% June, 27% August) and cyanobacteria (31% August), suggesting that their low levels in dreissenids represent ingested feces, which are being rejected. Increased shell size (12–27 mm; surrogate for age; 3 sets) corresponded to an increase in diatom biomarkers (from 60 to 73%) but a decrease in chlorophyte biomarkers (from 31 to 17%); biomarker concentrations also decreased significantly (−3000 pmol/g ww/mm) in the most offshore collection, near Buffalo (Oct. 2005).     

Phytoplankton Composition and Biomass in the Offshore Waters of Lake Erie: Pre- and Post-Dreissena Introduction (1983–1993)

Phytoplankton was collected in all basins of Lake Erie during 42 cruises during the spring and summer from 1983 to 1993—a period that spans the Dreissena mussel invasion. Two potential impacts of Dreissena on the phytoplankton community of the western, central, and eastern basins of Lake Erie were evaluated: Was selective feeding occurring as observed in Saginaw Bay and were reductions in biomass evident in the offshore regions of the three basins of Lake Erie? In the western basin, significant summer decreases in Chlorophyta, Bacillariophyta, Cyanobacteria, and total phytoplankton biomass were observed after Dreissena introduction. Similarly in the spring, Bacillariophyta and total phytoplankton biomass and chlorophyll a concentrations decreased significantly. Since several divisions of phytoplankton did not decrease in phytoplankton biomass in the western basin, and spring Cyanobacteria biomass increased significantly while other divisions decreased in biomass, selective feeding on the phytoplankton community was suggested. Where significant reductions in biomass were observed in the offshore waters of the western basin, they were approximately 50% of the reduction observed at the nearshore sites in Lake Erie by other workers.Dreissena impact on the phytoplankton community of the pelagic waters of the central and eastern basin appeared to be minimal. Pre- and post-Dreissena total phytoplankton biomass and chlorophyll a concentrations were not significantly different or increased significantly after the Dreissena invasion. Biomass of several divisions of phytoplankton significantly increased after Dreissena introduction in the central and eastern basins. These included Bacillariophyta (central basin), Cyanobacteria (central and eastern basin), Chrysophyta (eastern basin), Chlorophyta biomass (eastern basin) and phytoplankton biomass (central basin) and chlorophyll a (central basin) in the spring, and Chrysophyta (eastern basin) and Cryptophyta biomass (central basin) in the summer. Generally, a reduction in phytoplankton biomass would be expected as a result of Dreissena grazing, not an increase in biomass. Dreissena-mediated changes in phytoplankton have generally occurred in shallow, well-mixed lakes, ponds, and embayments, not in deeper waters such as the central and eastern basins of Lake Erie.     

Post-dreissenid Increases in Transparency During Summer Stratification in the Offshore Waters of Lake Ontario: Is a Reduction in Whiting Events the Cause?

Since the dreissenid invasion of the lower Great Lakes, calcium concentrations in the offshore waters of Lake Ontario have decreased by approximately 4–5 mg/L. This decline has coincided with a three-fold reduction in August turbidity values and nearly a doubling of Secchi depths, presumably due to reduced summer calcite precipitation events in the lake. The reductions in calcium have followed a dramatic reduction in alkalinity in the central and eastern basins of Lake Erie, which provides most of the inflow to Lake Ontario. This reduction in alkalinity in Lake Erie corresponds to a period of rapid dreissenid growth in that lake, strongly suggesting calcium uptake by dreissenid mussels as a causative factor. The mass of calcium resident in the dreissenid population in Lake Erie, estimated from published lake-wide census data, is sufficient to account for the observed decreases in alkalinity. In addition, observed changes in alkalinity in Lake Ontario closely match those expected to result from inflows from Lake Erie, based on mass balance considerations. Considered in sum, our data strongly suggest that calcium uptake by dreissenid mussels in Lake Erie has resulted in decreases in the calcium concentration in Lake Ontario, reducing the frequency and/or intensity of whiting events in the latter lake. We believe this is the first report of an increase in transparency that can be reasonably attributed to a chemical change brought about by Dreissena. These increases in transparency may have very different consequences than those of dreissenid filtration activities. For example, rather than decreasing phytoplankton populations, the improved light climate might increase summer phytoplankton populations, particularly sub-epilimnetic ones.     

Delivery of nutrients and seston from the Muskegon River Watershed to near shore Lake Michigan

Drowned river mouth lakes are major features of coastal Great Lakes habitats and may influence nutrient and organic matter contributions from watersheds to near shore coastal zones. In May through October 2003, we measured loads of nutrients, surficial sediment, and seston to track the delivery of riverine-derived materials from the lower Muskegon River Watershed (MRW) into the near shore area of southeast Lake Michigan. Nutrient flux data indicated that seasonal loads of 1800 metric tons (MT) of particulate organic carbon, 3400 MT of dissolved organic carbon, and 24 MT of total phosphorus were discharged from the lower Muskegon River, with approximately 33% of TP load and 53% of the POC load intercepted within the drowned river mouth terminus, Muskegon Lake. Carbon: phosphorus molar ratios of seston in Muskegon River (C:P = 187) and Muskegon Lake (C:P = 176) were lower than in Lake Michigan (C:P = 334), indicating phosphorus limitation of phytoplankton in near shore Lake Michigan. Isotopic signatures of seston collected in Muskegon Lake were depleted in δ¹³C (− 30.8 ± 1.6‰) relative to the isotope signatures of seston from Lake Michigan (− 26.2 ± 1.3‰) or the mouth of the Muskegon River (− 28.1 ± 0.5‰), likely due to the presence of biogenic methane in Muskegon Lake. Seston δ¹⁵N increased on a strong east-to-west gradient within Muskegon Lake, indicating significant microbial processing of nutrients. The extent of nutrient uptake in Muskegon Lake altered the chemical and isotopic characterization of seston flowing into Lake Michigan from Muskegon River.     

Improving fishery-independent indices of abundance for a migratory walleye population

The primary goal of many biological surveys is to provide an unbiased representation of trends of population abundance. However, there are often factors other than abundance that vary over time and influence catch rates and thus inferences about population trends from surveys. This is particularly true for highly mobile species because of interannual variation in the timing, extent, and duration of movements, and for surveys that are not randomized with respect to space and time. We developed general and generalized linear mixed models to standardize Canadian and United States fishery-independent surveys that provide an index of basin-level walleye (Sander vitreus) population abundance trends in Lake Erie (1983–2008). In Canadian waters, the probability of a non-zero catch was associated with the type of gill net set (canned > bottom sets), the presence of hypoxia (negative trend; −), and secchi depth (−). Positive catch rates were associated with the set type (canned > bottom) and water depth (+). In United States waters, survey catch rates were associated with secchi depth (−) and surface water temperature (+). For each case, the best model included random effects (interactions between year, week, basin, sub-basin) which accounted for a modest amount of the total variation. General abundance trends were similar between the standardized and nominal indices, but substantial annual variation in the direction and magnitude of the difference between indices was observed. We recommend the use of standardized indices for walleye population assessments because these account for factors influencing catch rates other than changes in abundance.