Phosphorus enrichment and carbon depletion contribute to high Microcystis biomass and microcystin concentrations in Ugandan lakes

Abstract

We investigated the factors influencing cyanobacterial biomass and microcystin (MC) concentrations in several Ugandan lakes from September 2008 to February 2009. We characterized thermal structure, light availability, nutrient concentrations, chlorophyll a, phytoplankton d 13 C (as an indicator of CO 2 limitation), and phytoplankton community composition and abundance as well as MC concentrations. We used these data to test several hypotheses based on previous research in temperate lakes regarding the factors that encourage high cyanobacterial biomass and MC concentrations. Site characteristics that appeared to favor high cyanobacterial biomass (especially Microcystis) included: high total phosphorus concentrations, low total nitrogen to total phosphorus (TN : TP) ratios, and possibly low CO 2 availability. Light availability, total nitrogen concentrations, and thermal structure of the water column were not related to cyanobacterial biomass. MC concentrations were strongly related to Microcystis biomass (and were not related to the biomass of any other cyanobacterial taxa), which was positively correlated with total phosphorus and chlorophyll a concentrations. MC cell content may be moderated by CO 2 availability, with MC cell quotas tending to be lower where the potential for C-limitation of photosynthesis was higher. In these phosphorus-rich tropical lakes, the shallowest study sites were most conducive to the development of large standing crops of Microcystis and high MC concentrations. The environmental conditions that appear to favor high cyanobacterial biomass and MC concentrations in our Ugandan study lakes are similar to what has been observed for temperate lakes.

Bioaccumulation and biomagnification of mercury in African lakes: The importance of trophic status

“…Lake Victoria). Many of these lakes can sustain perennially high standing crops of phytoplankton (Poste et al, 2013), offering a unique opportunity to explore the influence of lake trophic status on Hg uptake and trophic transfer, and in particular, to assess the potential for eutrophication-mediated biodilution (Pickhardt et al, 2002;Herendeen and Hill, 2004;Chen and Folt, 2005;Karimi et al, 2007) of Hg in tropical African lakes. Previous studies of Hg in the food webs of several sub-Saharan African lakes (including several embayments of Lake Victoria, Lake Malawi, Lake Tanganyika, Lake Chad and several smaller lakes; Table 1) have typically reported low Hg concentrations in fish (Black et al, 2011), and Hg biomagnification ( Table 1) that falls within the range encountered in freshwater and marine systems worldwide (Lavoie et al, 2013).…”

Tropical cyanobacterial blooms: a review of prevalence, problem taxa, toxins and influencing environmental factors

“…Flushing of water bodies was not analysed in this meta-analysis due to the lack of detailed information for flushing rates of different water bodies. Poste et al (2013) showed that higher flushing rates of Napolean Gulf in Lake Victoria had led to a lower biomass of Microcystis compared to other less flushed sites such as Lake George and Lake Saka in Uganda. However, the rate of flushing and difference in biomass was not quantified and thus, higher flushing in combination with other factors may have caused the lower biomass of Microcystis.…”

“…9). The lower microcystin concentration per cell in African lakes was based on a limited amount of studies and could be a result of the infrequent sampling and short time periods sampled (monthly for 1 year, Okello et al, 2010; monthly for 6 months, Poste et al, 2013). Data collected bimonthly and over a longer period of time should be more indicative of the trends.…”

“…Studies in temperate, sub-tropical as , 2004). The data on nutrient levels in tropical lakes and reservoirs are also sparse and thus the relationship beo n l y and reservoirs are also sparse and thus the relationship between N:P (nitrate:phosphate) ratios and microcystin proo n l y tween N:P (nitrate:phosphate) ratios and microcystin pro-well as tropical countries have found positive correlations between the concentration of microcystin and the concentration of cyanobacteria in blooms (Xu et al, 2008;Poste et al, 2013). Our analysis across tropical American and Asian regions also showed microcystins to significantly increase (Spearman's rank correlation, ρ=0.94, Pvalue=0.017*<P crit 0.05) with increasing concentration of Microcystis spp.…”

The Effect of Charophyte Communities on Phosphorus Flow in Lake Shinji, Japan

“…Although these nutrients are naturally supplied by soils to lake waters, industrial liquid waste from catchment basins, chemical fertilisers used on farms, and grey water from houses can also flow into the lakes, leading to over-nutrition. In particular, when the N/P ratio is lowered by an increase in phosphorus concentrations, the resulting cyanobacterial bloom typically causes drinking water supplied by the lake to have a foul odour (Poste et al, 2013;Ganguly et al, 2013). Although phosphorus is an essential nutrient in lake ecosystems, high concentrations can lead to over-nutrition, resulting in eutrophication and a reduction in water transparency.…”