Nutrient status and thermal stratification in Lake Victoria

Worldwide, human activity in the watershed has been found to induce lake responses at various levels, including at population and ecosystem scale. Recently, Carignan and Steedman (2000) reported on disruptions of biogeochemical cycles in temperate lakes following watershed deforestation and lor wildfire and Carignan et al., (2000 a, b) concluded that water quality and aquatic biota are strongly influenced by disturbances in the watershed. Similarly, Lake Victoria is no exception as people in its catchment have exploited it for the last hundred years or more, but have now begun to understand the extent to which they have thrown the lake into disorder and how their increasing activity in the watershed have driven some environmental changes within and around the lake

Nutrient limitation of phytoplankton in freshwater and marine environments: A review of recent evidence on the effects of enrichment

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/109760/1/lno1988334part20796.pd

Implications of changes in trophic diversity and food webs on fisheries and the environment

Most of the earth's ecosystems are experiencing slight to catastrophic losses of biodiversity, caused by habitat destruction, alien species introduction, climate change and pollution (Wilcove et al., 1998). These human effects have led to the extinction of native fish species, the collapse of their populations and the loss of ecological integrity and ecosystem functioning (Ogutu-Ohwayo & Hecky, 1991; Witte et al. , 1992a; Mills et al., 1994; Vitousek et al., 1996). Food webs are macro-descriptors of community feeding interactions that can be used to map the flow of materials and nutrients in ecosystems (Jepsen & Winemiller, 2002). Comparative food web studies have been used to address theoretical questions such as 'does greater trophic connectivity increase stability?' (Cohen et al., 1990), and 'does the number of trophic levels increase with productivity?' (Briand & Cohen, 1987). Answers to such questions have obvious applications for natural resources management. From a multi-species fisheries standpoint, there is a need to understand consumer-resource dynamics within complex trophic networks

The Relationship between Mixing and Stratification Regime on the Phytoplankton of Lake Bosomtwe (Ghana), West Africa

The seasonal changes in the phytoplankton community in terms of biomass composition and associated physicochemical parameters of the Lake Bosomtwe (Ghana) located in West Africa were studied between 2004 and 2006 to assess the mixing and stratification regime of the lake on the phytoplankton dynamics. From water samples obtained from a central index station, biomass composition was assessed by converting phytoplankton counts to wet weights-based approximation into cell volume values; whiles mixed layer and euphotic depths were analyzed using temperature and light profiles of the lake respectively. Total phosphorus was estimated using the Ascorbic Acid Method. Results from the dataset showed that the phytoplankton biomass was dominated by the Cyanophyceae throughout the study period despite the seasonal changes associated with the mixing and stratification regimes. There were significant inter-annual differences in the mean values of the euphotic depth and the wet weight biomass (P < 0.05). However differences in the mean values of the mixed layer depth, the ratio of the mixed layer depth:euphotic depth, and total phosphorus concentration (P > 0.05) were insignificant. High variations in the mixed layer depth (CV > 34 %) and the euphotic depth (CV > 32) drive similarly high variations in the wet weight biomass (CV > 28) as is the case for many stratifying tropical lakes. However, both were poor predictors of the phytoplankton wet weight biomass behaviour (mixed layer depth, r2 = 0.1034; euphotic depth, r

Seasonality of Primary productivity of phytoplankton of Lake Bosomtwe, Ghana -West Africa

The primary productivity and seasonality of phytoplankton and its seasonality in Lake Bosomtwe (Ghana), West Africa were studied from September 2005 to August 2006 using the dissolved oxygen method. The wet weight biomass (0.41±0.37 gCm-2d-1, n=25), Chlorophyll a (52.11±19.51 mg m-2, n=17), mixed layer depth (9.28±3.47 m, n=25), euphotic depth (4.43±1.76 m, n=25), ratio of mixed layer to euphotic depth (2.17±0.74, n=25), secchi disc depth (1.52±0.34 m, n=25), irradiance (1180±341.28 µE m-2s-1, n=25), extinction coefficient (0.97±0.46, n=25), and total phosphorus concentrations (1.84±0.49 µmol L-1, n=20) were measured to assess their influence on the productivity of the phytoplankton. Our assessment revealed high areal gross productivity of the phytoplankton (4.72 ± 1.56 gC m-2 d-1, n = 25) which is within the range for tropical African lakes, but concurrent high areal community respiration rates (4.34 ± 2.78 gC m-2 d-1, n = 25) contributed to a low net productivity (0.37 ± 2.32 gC m-2 d-1, n = 25) and growth rates (0.14 ± 0.33 d-1, n = 21). Variabilities in the gross productivity of the phytoplankton were high (CV =33.10 %) and driven by similarly high variabilities in the physicochemical and biological parameters. Productivity also exhibited clear seasonality associated with the mixing and stratification of the lake. The physicochemical and biological parameters (mixed layer depth, euphotic depth, secchi disc depth, irradiance, chlorophyll a, and the community respiration) all had a positive relationship with the primary production) whiles ratio of mixed layer to euphotic depth, total phosphorus, wet weigh biomass had a negative relationship with the gross productivity. Of all the measured parameters, only areal community respiration (r2 = 30.6 %) had a significant predictive value (r2 < 0.05).

The Nile perch invasion in Lake Victoria: cause or consequence of the haplochromine decline?

We review alternative hypotheses and associated mechanisms to explain Lake Victoria’s Nile perch takeover and concurrent reduction in haplochromines through a (re)analysis of long term climate, limnological and stock observations in comparison with size-spectrum model predictions of co-existence, extinction and demographic change. The empirical observations are in agreement with the outcomes of the model containing two interacting species with life-histories matching Nile perch and a generalized haplochromine. The dynamic interactions may have depended on size related differences in early juvenile mortality: mouth-brooding haplochromines escape predation mortality in early life stages, unlike Nile perch that have miniscule planktonic eggs and larvae. In our model predation on the latter by planktivorous haplochromine fry act as a stabilizing factor for co-existence, but external mortality on the haplochromines would disrupt this balance in favor of Nile perch. To explain the observed switch, mortality on haplochromines would need to be much higher than the fishing mortality that can be realistically re-constructed from observations. Abrupt concomitant changes in algal and zooplankton composition, decreased water column transparency, and widespread hypoxia from increased eutrophication most likely caused haplochromine biomass decline. We hypothesize that the shift to Nile perch was a consequence of an externally caused, climate triggered, decrease in haplochromine biomass and associated recruitment failure rather than a direct cause of the introduction

Basin-Scale Control on the Phytoplankton Biomass in Lake Victoria, Africa

The relative bio-optical variability within Lake Victoria was analyzed through the spatio-temporal decomposition of a 1997–2004 dataset of remotely-sensed reflectance ratios in the visible spectral range. Results show a regular seasonal pattern with a phase shift (around 2 months) between the south and north parts of the lake. Interannual trends suggested a teleconnection between the lake dynamics and El-Niño phenomena. Both seasonal and interannual patterns were associated to conditions of light limitation for phytoplankton growth and basin-scale hydrodynamics on phytoplankton access to light. Phytoplankton blooms developed during the periods of lake surface warming and water column stability. The temporal shift apparent in the bio-optical seasonal cycles was related to the differential cooling of the lake surface by southeastern monsoon winds. North-south differences in the exposure to trade winds are supported by the orography of the Eastern Great Rift Valley. The result is that surface layer warming begins in the northern part of the lake while the formation of cool and dense water continues in the southern part. The resulting buoyancy field is sufficient to induce a lake-wide convective circulation and the tilting of the isotherms along the north-south axis. Once surface warming spreads over the whole lake, the phytoplankton bloom dynamics are subjected to the internal seiche derived from the relaxation of thermocline tilting. In 1997–98, El-Niño phenomenon weakened the monsoon wind flow which led to an increase in water column stability and a higher phytoplankton optical signal throughout the lake. This suggests that phytoplankton response to expected climate scenarios will be opposite to that proposed for nutrient-limited great lakes. The present analysis of remotely-sensed bio-optical properties in combination with environmental data provides a novel basin-scale framework for research and management strategies in Lake Victoria

Mapping the spatial distribution of the biomass and filter‐feeding effect of invasive dreissenid mussels on the winter‐spring phytoplankton bloom in Lake Michigan

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/115963/1/fwb12653.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/115963/2/fwb12653_am.pd

Nutrients increase epiphyte loads: broad-scale observations and an experimental assessment

The original publication can be found at www.springerlink.comThere is a global trend towards elevated nutrients in coastal waters, especially on human-dominated coasts. We assessed local- to regional-scale relationships between the abundance of epiphytic algae on kelp ( Ecklonia radiata) and nutrient concentrations across much of the temperate coast of Australia, thus assessing the spatial scales over which nutrients may affect benthic assemblages. We tested the hypotheses that (1) percentage cover of epiphytic algae would be greater in areas with higher water nutrient concentrations, and (2) that an experimental enhancement of nutrient concentrations on an oligotrophic coast, to match more eutrophic coasts, would cause an increase in percentage cover of epiphytic algae to match those in more nutrient rich waters. Percentage cover of epiphytes was most extensive around the coast of Sydney, the study location with the greatest concentration of coastal chlorophyll a (a proxy for water nutrient concentration). Elevation of nitrate concentrations at a South Australian location caused an increase in percentage cover of epiphytes that was comparable to percentage covers observed around Sydney’s coastline. This result was achieved despite our inability to match nutrient concentrations observed around Sydney (<5% of Sydney concentrations), suggesting that increases to nutrient concentrations may have disproportionately larger effects in oligotrophic waters.Bayden D. Russell, Travis S. Elsdon Bronwyn M. Gillanders and Sean D. Connel