Print PageAnthropogenic activities can increase nutrient availability in aquatic systems, leading to eutrophication. However, much of our knowledge of eutrophication comes from temperate lakes. Lake Atitlán, Guatemala, is an endorheic tropical, volcanic lake experiencing increasing rates of nutrient inputs from wastewater inflows and agricultural runoff from the watershed. Phosphorus concentrations in the lake have increased during the last 30 – 40 years, shifting the lake away from an oligotrophic state. In 2008 and 2009, massive algal blooms formed in the lake at the end of the wet season. As many towns and settlements in the area rely on Lake Atitlán for drinking water, there is concern that these blooms may harbor toxin-producing cyanobacteria. From April – December 2010, we monitored nutrient availability and phytoplankton community composition. To determine if wastewater could be contributing to phytoplankton growth, we spiked sampled lake water with local wastewater. To determine which nutrients were limiting phytoplankton growth, we performed short-term enrichment bioassays during several different seasons.
Results/Conclusions
Wastewater significantly increased phytoplankton growth (F1,16 = 39.8, P <0.0001). Resource limitation was seasonally dynamic. In April, epilimnetic phytoplankton communities were co-limited by nitrogen and phosphorus, while meta- and hypolimnetic communities were slightly nitrogen-limited. N/P co-limitation remained through June, but by August, during the wet season, phytoplankton communities were phosphorus- or trace metal-limited. This shift corresponded with increased dominance of the phytoplankton by nitrogen-fixing, filamentous cyanobacteria. Secchi disk depth also decreased from April – December, but there were no phytoplankton blooms. While Lyngbya sp. and Microcystis sp. were also present throughout the study period, cyanotoxins were always below detection limits. Our results suggest that while phytoplankton biomass, nutrient limitation status, and community composition are dynamic, the lake is shifting toward eutrophy.
