Paleolimnological evidence of the effects of recent cultural eutrophication during the last 200 years in Lake Malawi,East Africa

To assess the long-term water quality changes in Lake Malawi, paleolimnological reconstructions of four radiometrically dated sediment cores collected in 1997/98 along a longitudinal transect of the lake were based on preserved diatom assemblages, as well as stratigraphies of organic carbon, total nitrogen, total phosphorus and biogenic silica concentrations. Population growth, deforestation and intensive agriculture, especially in the southern catchments, have accelerated soil erosion causing rivers to transport greater sediment and nutrient loads into Lake Malawi. Southern cores contain evidence of nutrient enrichment starting around 1940, as indicated by increased silica, carbon and nitrogen influxes. By 1980, increased rates of sedimentation, phosphorus influx, diatom influx, and relative abundances of eutrophic diatom taxa are attributable to accelerated nutrient enrichment accompanying soil erosion. The succession of diatoms in southern Lake Malawi begins with dominance by Aulacoseira nyassensis and Fragilaria africana, followed by a shift in 1980 towards Stephanodiscus, Cyclostephanos and Nitzschia, diatom taxa that have reduced silica requirements. Paleoecological results indicate that patterns of diatom assemblage change are not uniform lake-wide. Evidence of eutrophication extends to central Lake Malawi, but is not observable in the paleo-record from the deeper northern basin. The recent cultural eutrophication of the southern lake has impacted the biogeochemical cycling of silica, the available silica to phosphorus ratios, and the diatom communities of a large portion of Lake Malawi. These results provide a warning that eutrophication of this great lake is underway and will continue unless changes are made to current land use practices within the lake's catchment.     

Seasonal Si:C ratios in Lake Erie diatoms — Evidence of an active winter diatom community

Recent investigations of Lake Erie in the winter have demonstrated the occurrence of substantial phytoplankton communities largely consisting of the diatom Aulacoseira islandica (O. Müller) Simonsen. To assess the activity of this diatom community, multiple measures of production, both general and diatom-specific, were undertaken. We measured oxygen (O₂) evolution as proxy for carbon (C)-fixation and 2-(4-pyridyl)-5-((4-(2-dimethylaminoethylaminocarbamoyl) methoxy)-phenyl)oxazole (PDMPO) incorporation as a measure of silica (Si) deposition. The latter demonstrated conclusively that diatoms were active during winter months and confirmed that diatoms are the primary drivers of winter productivity. The stoichiometric relationship between carbon and silica in the winter Lake Erie phytoplankton assemblage was further compared to the activity of the summer community. Although the winter phytoplankton community was observed to be active, it was less active than the summer community, with lower measured rates of O₂ evolution and Si deposition. These findings provide a new and expanded understanding of the biological carbon production in Lake Erie.     

Burrowing mayfly (Ephemeroptera: Ephemeridae: Hexagenia spp.) bioturbation and bioirrigation: A source of internal phosphorus loading in Lake Erie

Traditional lake eutrophication models predict lower phosphorus concentrations with decreased external loads. However, in lakes where decreased external phosphorus loads are accompanied by increasing phosphorus concentrations, a seeming “trophic paradox” exists. Western Lake Erie is an example of such a paradox. Internal phosphorus loads may help explain this paradox. We examined bioturbation and bioirrigation created from burrowing mayfly, Hexagenia spp., as a possible source of internal phosphorus loading. Phosphorus concentrations of experimental microcosms containing lake sediments, filtered lake water, and nymphs (417/m²) collected from western Lake Erie were compared to control microcosms containing sediments and lake water over a 7-day period. Phosphorus concentrations in microcosms containing Hexagenia were significantly greater than microcosms without nymphs. Further, we estimate the soluble reactive phosphorus flux from the sediments due to Hexagenia is 1.03 mg/m²/day. Thus, Hexagenia are a source of internal phosphorus loading. High densities of Hexagenia nymphs in western Lake Erie may help explain the “trophic paradox.” Furthermore, Hexagenia may be a neglected source of internal phosphorus loading in any lake in which they are abundant. Future studies of phosphorus dynamics in lakes with Hexagenia must account for the ability of these organisms to increase lake internal phosphorus loading.     

Regression and Weighted Averaging Models Relating Surficial Sedimentary Diatom Assemblages to Water Depth in Lake Ontario

Diatom assemblages in surface sediment samples in depth profiles from Lake Ontario and from East Lake, a shoreline lake, were analyzed for the purpose of describing the relationship between species distribution and water depth. At both sites species composition varied markedly with sample depth. In East Lake a multiple regression of four habitat groups: benthic, epiphytic, tychoplanktonic, and euplanktonic, against water depth (30 cm–8 m) produced a relationship with a standard error (SE) of 1.5 m. An analysis of the ratio of euplanktonic diatoms to periphytic diatoms over a transect from 3 m to 150 m in Lake Ontario produced a multiple regression with an SE of 11.8 m. In another approach, water depth optima for 91 diatom taxa were developed using a weighted averaging (WA) technique. A good correlation (r² > 0.9) was found between measured and inferred water depth over the range 3 m to 30 m using a unimodal WA regression model. The transfer functions offer the possibility of inferring Holocene water level changes in Lake Ontario from fossil diatom assemblages in sediment cores.     

Spatio-temporal variability of diatom assemblages in surface sediments of Lake Peipsi

The modern diatom assemblages in surface sediments of large and shallow Lake Peipsi sensu stricto (2611 km²; maximum depth 12.9 m) in north-eastern Europe (Estonia) were studied. Ordination techniques were used to identify physical environmental parameters (grain size of sediment, water depth and distance from the shore) that explain the distribution of diatoms in the lake. Diatom species distribution and concentration in surface sediment showed large variations (concentrations varied from 3.3 × 10⁶ to 149 × 10⁶ g⁻¹ dry weight). The texture of surface sediments and concentration of diatoms are highly correlated (p ≤ 0.05), thus suggesting that the relocation of cohesive particles (silt) due to wind and wave actions affects the spatial variations of diatoms in surface sediments. The diatom data set analysed in the 1970s, and samples taken in 2006 showed some perceptible changes in the composition and concentrations of the diatom assemblages over this time period. However, main dominant diatom taxa have not changed during more than 25 years.     

Lake Erie's ecological history reconstructed from the sedimentary record

We evaluated the recent ecological history of Lake Erie from diatoms and geochemistry in sediment cores. Two major transition points in the ecology of the western basin (WB; 1985 and 2008) and central basin (CB; 1935 and 1982) were defined. Changes in abundance of diatom eutrophic indicators and geochemical markers were interpreted as a degradation in water quality after 1935 due to the effects of increased population, agriculture, and industrialization until abatement measures were enacted in the 1970s and 1980s. Diatom indicators suggested modest recovery from eutrophic conditions in Lake Erie, however diatom-inferred total phosphorus suggested that despite abatement efforts total phosphorus was not reduced below pre-impact levels. The effects on diatoms of increased temperature and dissolved silica also became apparent in the 1980s, and in the WB recent shifts were likely caused by increased pollution and recent climatic warming. Based on stratigraphic changes since 1985, the diatom trajectory suggests the phytoplankton of Lake Erie will likely remain in a state of flux for the near future due to a variety of countervailing impacts including unknown effects of mitigation efforts, legacy pollution, climate change, and changing upstream conditions.     

Current state of phytoplankton in the littoral area of Lake Baikal,spring 2017

To assess the current state of phytoplankton in the littoral area of Lake Baikal and provide a baseline for future comparisons, we sampled spring plankton communities from the 44 littoral and 3 pelagic stations covering all three basins of the lake. The study examined chemical parameters of water (NH₄⁺, NO₂⁻, NO₃⁻, PO₄⁻³, Si, COD), species composition, abundance, and biomass of phytoplankton in Lake Baikal during late spring 2017. Sharp spatial heterogeneity was observed in the distribution of phytoplankton biomass along the western (399 ± 72 mg/m³) and eastern (1319 ± 220 mg/m³) shores of the lake. The phytoplankton were diverse, with 79 species; dominant algae were different from site to site and from south to north throughout the lake. In Southern and Central Baikal, we recorded an intense bloom of the diatom Synedra acus subsp. radians (28–1400 cells/mL), similar to that observed for the past 10 years, while the chrysophyte Dinobryon cylindricum dominated in Northern Baikal. The diatoms Aulacoseira baicalensis, A. islandica, and Stephanodiscus meyeri that were dominant in the 1960s–1990s were not numerous in 2017 (0.5–10 cells/mL). This change in dominant species indicates structural changes in the phytoplankton of Lake Baikal, which have led to the disappearance of the main distinctive feature of the Baikalian phytoplankton – the alternation of extremely high (with the algal biomass over 1000 mg/m³) and extremely low (less than 100 mg/m³) productivity years. The ecological equilibrium appears to have shifted towards a new steady state.     

Analysis of Models and Measurements for Sediment Oxygen Demand in Lake Erie

In situ measurements of water column oxygen consumption and sediment oxygen demand (SOD) in the central basin of Lake Erie are historically 50% to 100% larger than observed from decreases in stocks of dissolved oxygen. Recent statistical and modeling analyses of observed oxygen data, and in situ measurements of SOD using a dome with gentle mixing, suggest consistent values for SOD of 0.2 to 0.3 g/m²/d. These values are lower than historical estimates. Three SOD models applicable to Lake Erie are examined. One (Walker) describes SOD as a function of oxygen concentration, and of biological and chemical components. The second (Klapwijk) describes SOD as a function of carbonaceous oxidation, nitrification, denitrification, and ammonia produced by diffusion and in the aerobic, anoxic, and anaerobic zones of the sediments. The third (DiToro) describes SOD as a function of settling fluxes of algae and organics from the water column, and assumes that all anaerobically produced carbon is oxidized in the sediments. It is suggested that aspects of the latter two models are required for future modeling of SOD in Lake Erie. More measurements of sediment profiles of POC are required to resolve certain modeling questions.     

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.     

Hydrological alterations as the major driver on environmental change in a floodplain Lake Poyang (China): Evidence from monitoring and sediment records

Due to the lack of long-term records on shallow lake environmental change, knowledge of the processes and mechanisms behind the limnological response of many shallow floodplain lakes to hydrological alterations and nutrient loading is often limited. We examined seasonal monitoring data and a dated sediment core from Lake Poyang, a large floodplain lake located on the Yangtze floodplain in the SE China. Multivariate analysis based on contemporary data (diatoms and water quality) revealed that the seasonal changes in the diatom assemblage of the lake were correlated with water temperature and Secchi depth (SD), although the weak spatial effect was not negligible. During the dry winter season, low water temperature, low SD, and high nutrient levels, were accompanied by high abundances of planktonic Aulacoseira species along with Stephanodiscus hantzschii, a species well adapted to cold and eutrophic waters. During the summer wet season, however, when water temperature and SD were high and nutrient levels low, benthic and epiphytic diatoms, such as the genus Achnanthes, dominated. Sediment records of diatoms and geochemistry were used to estimate long-term variation in the ecological condition of the lake. During the past ~60 years, the lake has shifted from a natural hydrologically connected, oligotrophic lake dominated by benthic and epiphytic diatoms to a poorly hydrologically connected, eutrophic state driven by nutrient-tolerant planktonic and eutrophic diatoms. Furthermore, our results indicate that the proposed Poyang dam may severely affect the water quality and ecosystem of the lake by altering its seasonal hydrology.     

Potential Strategies for Recovery of Lake Whitefish and Lake Herring Stocks in Eastern Lake Erie

Lake Erie sustained large populations of ciscoes (Salmonidae: Coregoninae) 120 years ago. By the end of the 19^th century, abundance of lake whitefish (Coregonus clupeaformis) had declined drastically. By 1925, the lake herring (a cisco) population (Coregonus artedii) had collapsed, although a limited lake herring fishery persisted in the eastern basin until the 1950s. In the latter part of the 20^th century, the composition of the fish community changed as oligotrophication proceeded. Since 1984, a limited recovery of lake whitefish has occurred, however no recovery was evident for lake herring. Current ecological conditions in Lake Erie probably will not inhibit recovery of the coregonine species. Recovery of walleye (Sander vitreus) and efforts to rehabilitate the native lake trout (Salvelinus namaycush) in Lake Erie will probably assist recovery because these piscivores reduce populations of alewife (Alosa psuedoharengus) and rainbow smelt (Osmerus mordax), which inhibit reproductive success of coregonines. Although there are considerable spawning substrates available to coregonine species in eastern Lake Erie, eggs and fry would probably be displaced by storm surge from most shoals. Site selection for stocking or seeding of eggs should consider the reproductive life cycle of the stocked fish and suitable protection from storm events. Two potential sites in the eastern basin have been identified. Recommended management rocedures, including commercial fisheries, are suggested to assist in recovery. Stocking in the eastern basin of Lake Erie is recommended for both species, as conditions are adequate and the native spawning population in the eastern basin is low. For lake herring, consideration should be given to match ecophenotypes as much as possible. Egg seeding is recommended. Egg seeding of lake whitefish should be considered initially, with fingerling or yearling stocking suggested if unsuccessful. Spawning stocks of whitefish in the western basin of Lake Erie could be utilized.     

The Distribution of Diatoms Near a Thermal Bar in Lake Baikal

Studies of the spatial distribution of suspended matter and of diatoms across a thermal bar in Lake Baikal revealed that waters sinking at the thermal bar front (2 km off-shore) move to the bottom along the underwater slope, and may be traced to a depth of at least 700 m, 5 km off-shore. The distributions of 11 diatom species across the thermal bar were significantly different, reflecting their different ecological requirements. Maximum concentrations of Asterionella formosa, Nitzschia acicularis, Aulacoseira islandica occur near the shore. The distribution of Stephanodiscus hantzschii, a species characteristic of the Selenga River, suggests that riverine waters penetrate far into the lake along its eastern shore. Another species typical of the Selenga River, Stephanodiscus minutulus, was found both near the shore, and in the surface waters of the open lake. Endemic spring baikalian diatoms Aulacoseira baicalensis and Cyclotella baicalensis were found at highest concentrations in the deep waters far off-shore, beyond the thermal bar. Another endemic diatom, the autumn species Cyclotella minuta was evenly distributed over the open lake at all depths. The data obtained shed new light on the dynamics of water masses near thermal bars, and on the ecology of the diatom species studied.     

Reconstruction of lake-level and climate changes in Lake Qarun,Egypt, during the last 7000 years

Sediment cores from Lake Qarun provide a record of mid-late Holocene climatic changes in Northern and Eastern Africa as well as environmental changes due to the activities of ancient Egyptians. We used sedimentological, mineralogical, and geochemical analyses of the cores to investigate long-term variations in lake level due to changing hydrologic inputs. An age model based on three paired ¹⁴C and paleomagnetic measurements suggests that the base of the sediment cores is as old as ∼ 5000 B.C.E. Geochemical analyses indicated that lake sediments were derived from Nile floods with an admixture of Saharan sand. Laminated endogenic carbonate-rich clayey silt lithofacies with benthic diatoms are indicative of relatively low lake levels, saline waters and dry conditions; massive lithofacies with planktonic diatom species are indicative of relatively high lake levels, fresh waters and humid conditions. Faintly laminated clayey silt lithofacies suggest intermediate conditions. Variations in lithology as well as diatom composition suggest that the lake level has varied from relatively high levels in its early history to lower levels in later years although there have been numerous cycles in water level over the past 7000 years. A combination of climate changes in the source area of the Nile River as a result of monsoon dynamics; climatic changes in the setting area of the Lake Qarun; and human activities through the dynasties in Egypt produced these variations in lake level.     

Biomarkers of algal populations in phytoplankton,filamentous algae,and sediments from the eastern basin of Lake Erie 2003–2005

Algal carotenoids (n:16) and chlorophylls (n:7) were determined in phytoplankton (n:60), filamentous algae (n:14) and sediments (n:44), collected from two nearshore sites in eastern Lake Erie (Van Buren Point (VBP), 6.5 m; Point Gratiot (PG), 17 m), during summer and fall of 2003, 2004 and 2005. The most prominent biomarkers for diatoms (fucoxanthin), cryptophytes (alloxanthin), chlorophytes (chlorophyll-b) and cyanobacteria (zeaxanthin) revealed temporal variations in phytoplankton community composition, which were correlated to water temperatures: i) seasonal succession, from diatoms with some cryptophytes in June (cool), to an increased percentage of chlorophytes and cyanobacteria in August and September (warm); ii) differences between 2004 (cool; cryptophytes more abundant) and 2005 (warm; chlorophytes more abundant). Filamentous algae (chlorophytes, epiphytic diatoms; some cyanobacteria) varied in condition, according to levels of chl-a, pheopigments, and class biomarkers: high, at VBP (growing; decaying; bont/E suspect); low, at PG (mostly dead). Relative to phytoplankton, sediments were depleted in several biomarkers (chl-a; diadino-, neo- and violaxanthin) but enriched with others, particularly at PG (pheopigments; diatoxanthin; canthaxanthin, echinenone; alloxanthin). Sediment composition was characterized by strong differences between sites (chl-a, chl-b and fucoxanthin nearly 10-fold greater at VBP than PG) and increasing accumulation of biomarkers from year-to-year. A linear log–log function, relating total carotenoids to total chlorophylls (intercept −0.516, slope 1.054, r² 0.96), implied increased biodegradation among specimen types: levels of pigments (pmol/g ww) decreased three orders of magnitude, from phytoplankton, through filamentous algae, to sediments. Deviation from a 1:1 relationship indicated 1.7-fold depletion of carotenoids relative to chlorophylls at mid-range.     

Heavy metals in sediments and uptake by burrowing mayflies in western Lake Erie basin

During the past two decades, burrowing Hexagenia mayflies have returned to the western basin of Lake Erie. Because of their importance as a prey resource for higher trophic levels and their extensive residence time in potentially contaminated sediment, Hexagenia may be a source of heavy metal transfer. To better understand the distribution and transfer of heavy metals in sediment and mayflies, sediment and mayfly nymphs were collected from 24 locations across the western basin of Lake Erie in May 2007. Following USEPA protocols, samples were analyzed for 16 elements using ICP-OES or ICP-MS. Metal concentrations in the sediments exceeded the Threshold Effect Level for at least one metal at all sample sites. Sediment heavy metal distribution profiles indicate metal concentrations are correlated with organic matter content, and the highest heavy metal concentrations were found in the central deeper region of the western basin where organic content in the sediments was greatest. Hexagenia were distributed throughout the western basin, with greatest density (1350/m²) within the Detroit River plume. The Cd and Zn levels in mayflies were on average approximately 4 and 2 times greater, respectively, than sediment levels, and the Cd concentrations in the sediments exceeded the Threshold Effect Level at 27 of 28 sites and exceeded the Probable Effect Level at 9 of 28 sites. Spatial representation of heavy metal concentrations in mayflies exhibited a similar pattern to the spatial distribution of heavy metals and organic matter in the sediments with higher concentrations of metals found in mayflies residing in the central deeper region of the western basin.     

Polymethylene-interrupted fatty acids: Biomarkers for native and exotic mussels in the Laurentian Great Lakes

Freshwater organisms synthesize a wide variety of fatty acids (FAs); however, the ability to synthesize and/or subsequently modify a particular FA is not universal, making it possible to use certain FAs as biomarkers. Herein we document the occurrence of unusual FAs (polymethylene-interrupted fatty acids; PMI-FAs) in select freshwater organisms in the Laurentian Great Lakes. We did not detect PMI-FAs in: (a) natural seston from Lake Erie and Hamilton Harbor (Lake Ontario), (b) various species of laboratory-cultured algae including a green alga (Scenedesmus obliquus), two cyanobacteria (Aphanizomenon flos-aquae and Synechococystis sp.), two diatoms (Asterionella formosa, Diatoma elongatum) and a chrysophyte (Dinobryon cylindricum) or, (c) zooplankton (Daphnia spp., calanoid or cyclopoid copepods) from Lake Ontario, suggesting that PMI-FAs are not substantively incorporated into consumers at the phytoplankton-zooplankton interface. However, these unusual FAs comprised 4-6% of total fatty acids (on a dry tissue weight basis) of native fat mucket (Lampsilis siliquoidea) and plain pocketbook (L. cardium) mussels and in invasive zebra (Dreissena polymorpha) and quagga (D. bugensis) mussels. We were able to clearly partition Great Lakes' mussels into three separate groups (zebra, quagga, and native mussels) based solely on their PMI-FA profiles. We also provide evidence for the trophic transfer of PMI-FAs from mussels to various fishes in Lakes Ontario and Michigan, further underlining the potential usefulness of PMI-FAs for tracking the dietary contribution of mollusks in food web and contaminant-fate studies.     

A paleolimnological assessment of human impacts on Lake Superior

To understand environmental conditions in Lake Superior over the last two centuries, we conducted a paleolimnological study on two sediment cores collected in the eastern and western regions of the lake. We examined the diatom fossil assemblages, sedimentation rates, organic and trace metal accumulation rates, and GIS-reconstructed human land use stressors in order to evaluate lake history and the impacts of human activities. There is evidence that the diatom community reorganized due to nutrient enrichment beginning around the time of European settlement and significant agricultural development. Trace metal profiles tracked a period of mining and ore processing which temporarily increased trace metal loads to the lake in the mid- to late-20th century. In recent decades, more oligotrophic diatom species were favored, suggesting nutrient decreases associated with remedial activities. The diatom community is now dominated by the Cyclotella comensis complex, suggesting changes in the lake's physical and chemical processes associated with climate change. Similar long-term environmental trends were observed in both core locations, but the timing of some events differed, indicating localized effects such as nutrient enrichment. An understanding of Lake Superior's past responses to human activities can inform management decisions that account for influences within and outside the lake's catchment.     

Distribution and geochemical processes of boron in the multimedia of Lake Qinghai,China

Lake Qinghai on the Qinghai-Tibet plateau is the largest lake in China. This study investigated the concentration and geochemical processes of boron (B) in lake water, lake sediment and river samples collected from Lake Qinghai and the Buha River. In addition, lake sediment pore water samples were analyzed. The concentrations of B and major ions, including K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl^? and SO₄^2?, were analyzed in all the water samples. The average concentration of B was 0.07?mg?L^?1, 6.37?mg?L^?1, 12.79) mg?L^?1 and 59.42?mg?kg^?1 for river water, lake water, pore water, and sediment, respectively. There were significant (p?<?0.05) and positive (r?=?0.70) relationships between the B concentrations in bottom water of the lake and in lake sediment, indicating that B diffusion from the sediment plays an important role in the concentration of B in bottom lake water. The differences in B concentrations and B/major ion molar ratios of the river water, lake water, and pore water indicated the following geochemical processes: 1) B is enriched in the lake water through evaporative concentration; 2) B is removed from the lake water through mineral precipitation as well as sorption onto colloids; 3) Solid-phase B in sediments was released through dissolution driven by organic matter mineralization. B/Cl and Na/Cl molar ratios alone are not enough to identify the sources of B in the water of inland closed-basin saline lakes because of these processes.     

Distribution of Major and Trace Elements in Sediments and Pore Water of Lake Erie

Concentration profiles of major and trace elements were determined in sediment cores from the Central Basin of Lake Erie. The concentrations of trace elements (As, Cd, Cr, Cu, Hg, Mn, Ni, Pb, and Zn) in the sediments were greatest at approximately 18 cm sediment depth, corresponding to the early 1960s. The concentrations of all elements in the sediments have considerably decreased in the last decade. However, the concentrations of Hg, Pb, and Cd are still about 10, 5, and 4 times greater, respec, than those in the pre-industrial sediments. The profiles of dissolved concentrations of the trace ele in pore water were characterized by maxima below the sediment-water interface, dropping off rapidly within the zone of reduced sediments. Concentrations of dissolved trace elements below the sediment-water interface are about one order of magnitude greater than those in the lake water. Conservative estimates of benthic fluxes ranged from 0.04 /μg/cm².y to 194 /μg/cm².y for Cd and Fe, respectively. Upward diffusive remobilization from sediments to lake water is a significant transport process in the Central Basin of Lake Erie and may play an important role in the transport of trace elements from the sediments.     

Phytoplankton trends in the Great Lakes, 2001–2011

We describe recent trends in phytoplankton composition and abundance in the Laurentian Great Lakes using synoptic spring (April) and summer (August) sampling events from 2001 through 2011, a period of rapid shifts in pelagic food webs and water quality. Data analysis identified qualitative and quantitative changes in algal densities, biovolume, and taxonomic composition of assemblages. Since 2001, Lake Superior has changed subtly with an increase in small-celled blue-green algae in spring and a recent decline in summer centric diatoms, possibly a result of lake warming and changes in water quality. Spring phytoplankton declines mainly attributed to diatoms occurred in Lakes Huron and Michigan, a probable result of invasions by non-native dreissenids that have reduced pelagic nutrients and selectively consumed certain taxa. The decline in Lake Huron's spring phytoplankton biovolume was earlier and more severe than that in Lake Michigan, despite a faster and more abundant dreissenid invasion in Lake Michigan. Lake Erie's central basin had a notable increase in spring centric diatoms (largely Aulacoseira), while the whole of Lake Erie shows a summer increase in cyanobacteria, complementing that found in coastal regions. The composition of Lake Ontario's species assemblage shifted, but little overall change in algal abundance was observed with the exception of higher summer densities of cyanophytes. Additional mechanisms for shifts in the pelagic primary producers are described or hypothesized in the context of concurrent shifts in water quality and invertebrate populations. Tracking these trends and explaining driving factors will be critical to the management of lake conditions.