Harvey A. Bootsma

Great Lakes Cladophora in the 21st Century: Same Algae-Different Ecosystem

ABSTRACT Nuisance growth of the attached, green alga Cladophora was considered to have been abated by phosphorus management programs mandated under the Great Lakes Water Quality Agreement. The apparent resurgence of nuisance growth in Lakes Erie, Michigan and Ontario has been linked conceptually to ecosystem alterations engineered by invasive dreissenid mussels (Dreissena polymorpha and Dreissena bugensis). Here, we apply contemporary modeling tools and historical water quality data sets in quantifying the impact of long-term changes in phosphorus loading and dreissenid-mediated changes in water clarity on the distribution and production of Cladophora. It is concluded that reductions in phosphorus loading in the predreissenid period achieved the desired effect, as model simulations were consistent with the biomass declines reported from the early 1970s to the early 1980s. These declines were, however, largely offset by dreissenid-driven changes in water clarity that extended the depth of colonization by Cladophora, increasing total production. We were not able to isolate and quantify the significance of dreissenid mediation of phosphorus cycling using the historical database. Phosphorus management remains the appropriate mechanism for reducing nuisance levels of Cladophora growth. The development of action plans will require an improved understanding of nearshore phosphorus dynamics such as might be obtained through regular monitoring of soluble reactive phosphorus levels, internal phosphorus content and Cladophora biomass in impacted nearshore regions of the Great Lakes.

Mercury Concentrations in the Food Web of Lake Malawi, East Africa

Little is known about the concentrations of mercury (Hg) and factors affecting this contaminant in tropical freshwater biota. Lake Malawi, an East African Rift Valley lake, is internationally renowned for having the highest diversity of fish species in the world, and the lake supports subsistence fisheries for the three riparian countries. In this study the concentrations of Hg in fish and invertebrates collected from the lake in 1996 and 1997 were examined, and tissue stable carbon (δ13C) and nitrogen (δ15N) isotopes were used to distinguish and contrast those consumers relying upon carbon fixed by either benthic or pelagic primary producers, and their trophic positioning. Concentrations of Hg were generally low (2 to 200 ng/g wet weight) in fish from Lake Malawi. In addition, pelagic fish contained significantly higher concentrations of Hg than the benthic species. As in temperate systems, log-transformed Hg concentrations were significantly predicted by δ15N and the highest concentrations of Hg were found in the largest fish within each species. The rate of Hg accumulation was not significantly different between the pelagic and benthic fishes but pelagic species had significantly higher Hg concentrations at a common weight and trophic position. The slope of the Hg-δ15N relationship in Lake Malawi was comparable to what has been found in temperate and arctic lakes, suggesting that Hg accumulation in freshwater food webs is independent of climatic factors and species composition.

Impact of Land Use on Sediment and Nutrient Yields to Lake Malawi/Nyasa (Africa)

Thirteen rivers flowing into Lake Malawi/Nyasa from the country of Malawi in southern Africa were studied over 1 year to estimate annual yields of sediments and nutrients from the terrestrial catchments to the lake. The river basins ranged in size from 113 to 12,110 km2, but maximum elevations above lake level were similar in all the catchments yielding quite different mean basin slopes. Flow was seasonally variable with low or no flow at the end of the prolonged dry season. Annual runoff varied from 37 to 617 mm with most rivers having below average discharges in 1997. The river basins were all affected to varying degrees by subsistence farming which is the dominant land use in Malawi. Sediment and nutrient (C, N, P, Si) concentrations, loads, and yields were sensitive to the degree of agricultural land use in the basins. Lowest sediment concentrations, loads, and yields were characteristic of the small, steep catchments with a high proportion of intact forest. Highest concentrations, loads, and yields of sediments were from densely settled catchments with extensive agricultural activity. Concentrations, loads, and yields of nutrients tended to follow the particulate fluxes as dissolved components were less responsive to basin disturbance. Increasing populations and associated agricultural development may already have increased nutrient loading to Lake Malawi by 50% as forests have been increasingly cleared for agriculture.

A Comparative Introduction to the Biology and Limnology of the African Great Lakes

Abstract The East African rift valley region contains the earths largest aggregation of tropical lakes. Three of these lakes—Victoria, Tanganyika, and Malawi—hold one quarter of the earths total surface freshwater supply, and are home to a myriad of fish species. Apart from the diversity and endemicity of their biota, properties that distinguish the African Great Lakes from their North American counterparts include their great age, long sedimentary records, long residence times, persistent stratification, continuously warm temperatures at all depths, major ion composition, and a propensity for nitrogen limitation. Current management problems include over-fishing, increased input of sediment and nutrients, and in the case of Lake Victoria, loss of endemic fish species and the proliferation of the introduced water hyacinth. As in the Laurentian Great Lakes basin, the harmonization of research programs and management strategies among the various riparian countries is a challenge. While research activities on all three lakes have increased in the last decade, there remains a need for integrated, multidisciplinary research in order to develop conceptual and numerical models that provide insight into the functioning of large, tropical, freshwater aquatic ecosystems. Particular issues that may be pursued most profitably in the African Great Lakes include the links between climate and biogeochemical cycles, the role of biodiversity in ecosystem functioning, and paleoclimate reconstruction over millions of years.

Nutrient Limitation of Heterotrophic Bacteria in Florida Bay

We examined heterotrophic bacterial nutrient limitation at four sites in Florida Bay, U. S. in summer 1994 and winter 1995. Bacterial growth and biomass production in this system were most limited by inorganic phosphorus (P) in the eastern and southern regions of the bay. Nutrient additions stimulated productivity and biomass accumulation mostly in summer. The magnitude of growth responses (thymidine incorporation) to nutrient additions was nearly an order of magnitude less in winter than summer. Biomass-normalized alkaline phosphatase activity in the northeast and south-central region was 5–20 times greater than in the northwest and north-central regions, suggesting that P is most limiting to planktonic growth in those areas. Chlorophyll levels were higher in the northwest and north-central regions and P-uptake into particles >1 μm, primarily phytoplankton, was also higher in these regions. Consistent with these observations, others have observed that P is advected into the bay primarily in the northwestern region. Abundant seagrasses in Florida Bay may promote heterotrophic bacterial production relative to phytoplankton production by releasing dissolved organic carbon that makes bacteria more competitive for limiting quantities of inorganic phosphate, especially in the eastern bay where turbidity is low, P is most limiting, and light levels reaching the benthic plants are high.

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 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...

Nearshore energy subsidies support Lake Michigan fishes and invertebrates following major changes in food web structure

Aquatic food webs that incorporate multiple energy channels (e.g., nearshore benthic and pelagic) with varying productivity and turnover rates convey stability to biological communities by providing independent energy sources. Within the Lake Michigan food web, invasive dreissenid mussels have caused rapid changes to food web structure and potentially altered the channels through which consumers acquire energy. We used stable C and N isotopes to determine how Lake Michigan food web structure has changed in the past decade, coincident with the expansion of dreissenid mussels, decreased pelagic phytoplankton production, and increased nearshore benthic algal production. Fish and invertebrate samples collected from sites around Lake Michigan were analyzed to determine taxa-specific 13C:12C (delta13C) and 15N:14N (delta15N) ratios. Sampling took place during two distinct periods, 2002-2003 and 2010-2012, that spanned the period of dreissenid expansion, and included nearshore, pelagic and profundal fish and invertebrate taxa. The magnitude and direction of the delta13C shift indicated significantly greater reliance upon nearshore benthic energy sources among nearly all fish taxa as well as profundal invertebrates following dreissenid expansion. Although the mechanisms underlying this delta13C shift likely varied among species, possible causes include the transport of benthic algal production to offshore waters and increased feeding on nearshore prey items by pelagic and profundal species. delta15N shifts were more variable and of smaller magnitude across taxa, although declines in delta15N among some pelagic fishes suggest a shift to alternative prey resources. Lake Michigan fishes and invertebrates appear to have responded to dreissenid-induced changes in nutrient and energy pathways by switching from pelagic to alternative nearshore energy subsidies. Although large shifts in energy allocation (i.e., pelagic to nearshore benthic) resulting from invasive species appear to affect total production at upper trophic levels, changes in trophic structure and utilization of novel energy pathways may help to stabilize food webs following species invasions.

Inputs, Outputs, and Internal Cycling of Silica in a Large, Tropical Lake

Abstract A silica budget was constructed for Lake Malawi by measuring major cycling processes, including river input/output, atmospheric deposition, sinking, burial, and vertical flux within the water column. A large proportion (> 80%) of the annual input to the lake occurred in the December to March rainy season. During this period, biogenic Si (BSi) made up 61% of Si input from rivers. Microscopic examination indicated that a large fraction of BSi was phytoliths, which likely originated from cultivated maize and terrestrial grasses. Over an annual cycle, river input made up approximately 25% of total Si input to the epilimnion, while 75% resulted from vertical exchange with Si-rich deep water. Due to its long hydraulic residence time, Lake Malawi retains nearly all of the Si that enters it. Despite high temperatures, a relatively high proportion (7 to 11%) of diatom production is permanently buried. A Si flux model is used to demonstrate that river discharge and vertical mixing can have similar effects on BSi burial rate over short time scales (one to two decades), but very different effects over longer time periods. This temporally dependent response must be taken into account when interpreting sediment records.

Ventilation of Lake Malawi / Nyasa

A tracer study was conducted on lake Malawi/Nyasa, one of the deepest and largest lakes in the world, in order to quantify the renewal rates of the deep water. For this purpose, concentrations of the anthropogenic trace gas chlorofluorocarbon-12 (CFC-12) were measured in water samples which were collected in glass ampoules and analyzed by a new gas chromatographic separation method. Based on measurements of stored duplicate samples, we conclude that the first-order degradation rate for CFC-12 in anoxic water of Lake Malawi/Nyasa lies in the range 0 to 0.012 yr−1. The tracer measurements are used in a 3-box mixing model from which average exchange times between the hypolimnion and the metalimnion of 18.5 years and 15.9 years are calculated for the cases of no degradation and maximum degradation in anoxic water, respectively. These exchange times are 2.7 to 2.2 times higher than have been estimated previously based on tritium measurements in 1976 by Gonfiantini and coworkers. The exchange times between the metalimnion and the epilimnion are calculated to be 3.7 years and 3.4 years, again for no degradation and maximum degradation, respectively. These exchange times are comparable to those estimated previously. Volumetrically averaged apparent CFC-12 ages of 8.9 and 21.1 years were calculated for the metalimnion and the hypolimnion, respectively, under the assumption of no degradation. Latitudinal gradients in the CFC-12 and dissolved oxygen concentrations on isopycnal surfaces suggests that the deep water originates predominantly in the southern part of the lake.