Site specific considerations 73

4.4.1. Main Lake basin versus Crescent Island basin

Chl-a levels in the Crescent Island basin were high, but almost always lower than in the Main Lake basin. The major exception was during an approximate five week period from October to November where levels were considerably higher than the Main Lake. A similar pattern was observed with phycocyanin levels and chl-c and the sharp increases in those pigments were due to dominance of the cyanobacterium Anabaena and the dinoflagellate Ceratium, respectively. Despite the large increase in biomass, qualitative observations of the surface water indicated no visible algal bloom. The in situ chlorophyll fluorescence vertical profile (Appendix B) suggested that the large biomass was sitting approximately 1 m below the surface of the water, a depth that could not be seen from the surface of the water. Therefore, although it was predicted that the Crescent Island basin would be less likely to support a cyanobacterial bloom compared to the Main Lake basin, which is not entirely true. While the decreased wind-induced mixing would prevent distribution of nutrients, the potential for more stability and vertical stratification give some algae—such as the buoyant Anabaena and flagellated Ceratium—a chance to optimize their position in the water column and bloom below the surface. This is important because the Crescent Island basin is often referred to as an undisturbed and pristine area of the lake, but in fact certain water quality issues may be present similar to the Main Lake, but just not always directly observable.

Although not measured in this study, Anabaena is able to produce the neurotoxins anatoxin-a (Sivonen and Jones 1999; Carmichael, 2001) and saxitoxin—well known as a paralytic shellfish poison (Humpage at al., 1984; Al-Tebrineh at al., 2010)—which can also pose health risks. Another feature of this Anabaena is its ability to fix atmospheric

nitrogen into usable forms. The dominance of Anabaena at certain times in Crescent Island may indicate periods of more limited nitrogen availability (Paerl et al., 2001). Since inflow from the Malewa largely only affects the Main Lake basin, and the majority of the Crescent Island basin border is relatively undeveloped, the main source of nutrients into the basin may be different from the Main Lake basin. For example there may be a greater influence due to re-suspension of nutrients from the swamp area leading to the Crescent Island basin that easily dries out. The outflow and waste from the defunct sewage treatment in the Northwest corner of the lake, enters into that shallow swampy area and may influence nutrients in the Crescent Island basin more than the Main Lake basin.

Particulate microcystin levels in the Crescent Island basin were almost always lower than all of the Main Lake sites on a given day (Figure 3.16a), indicating that the health risk of drinking water in the Crescent Island basin is even lower than the Main Lake basin. However, the potential for increased toxin production due to a Microcystis bloom was observed in the shoreline bloom that occurred in the basin on the morning of May 19, 2011. Compared to the regular sample from the middle of the Crescent Island basin the same day, particulate microcystins levels were 60 times greater in that shoreline bloom, although the level was still very low at 0.03 µg/L (Figure 3.16a).

4.4.2. Main Lake basin sites

A challenge was identified when comparing the various sites of the Main Lake basin due to the heterogeneity of the bloom. Although the lake is well mixed, the distribution of the bloom was not equal at all the sites, and from week to week there was also variability between where the highest biomass was found. This was due to the fact that algae move around the lake with wave action, wind and turbulence and it was impossible to separate those influences from specific site characteristics that might have actually been influencing biomass and toxins in this study. Unfortunately, although more detailed and intense sampling sites were added during Field Season 2, the lack of cyanobacteria and/or high biomass made it difficult to observe any relationships. However, there are a few points to make about two of the sites:

Near Malewa River Inflow (Site D)

This was the only site in the Main Lake basin that was consistently different from the others and this difference can be attributed to the constant inflow from the Malewa River. The water was generally more brown and turbid due to the loading of dissolved and particulate matter, and algal and cyanobacterial biomasses were overall lower. Despite the consistently low phycocyanin, particulate microcystin levels were widely variable at this site, ranging from 0.02-0.08 µg/L. Therefore the inflow seems to impact how well cyanobacteria can establish at this site, but other factors may be impacting the microcystin production.

Kamere public water collection area (Site M)

This is an area right on the southern shore where community members are often found filling up large jugs of water, whether it be for personal use or to sell. This site was only directly sampled during Field Season 2, although it is in close proximity to Site L (Kamere Beach) that was sampled during both field seasons. Although no cyanobacterial bloom was present during Field Season 2 biomass levels and community composition at this site were similar to the other sites in the Main Lake and it is recommended that this site continue to be monitored.