Water quality is an important aspect of a lake ecosystem, and can be degraded by nutrient loading of phosphorous and nitrogen that results from anthropogenic land use (Beaver et al. 2014). This eutrophication can lead to Harmful Algal Blooms (HAB’s) of potentially toxic phytoplankton known as cyanobacteria. Cyanobacteria are the most problematic, widespread nuisance algae and possess physiological traits like nitrogen-fixation and luxury consumption of phosphorous that allows exploitation of nutrient-deficient and enriched environments (Paerl et al. 2001). HAB’s can produce an array of organic compounds that can be toxic to zooplankton (Paerl et al. 2001), and this toxic effect can be a causative agent of an altered food chain.
In the plankton community, zooplankton rely on phytoplankton as their food source, and organisms of higher trophic levels rely on zooplankton as their source of energy (Bownik 2013). HAB’s have the potential to dominate the phytoplankton community and eliminate non-toxic phytoplankton species (Paerl and Huisman 2009) that zooplankton rely on for energy (Christofferson 1996). This can lead to a decline in zooplankton diversity, which promotes alterations in the food web. Man-made nutrient inputs have become increasingly common and have accelerated the eutrophication of lakes, leading to surges in phytoplankton, which can be predominantly composed of toxic cyanobacteria (Beaver et al. 2014). In our study we investigated whether or not there is a direct correlation between zooplankton species diversity and the presence or absence of HAB’s. We found a significant decline in zooplankton species diversity in a lake with an HAB compared to a lake with no such bloom.
Assessments were performed on two lakes in Northern Kentucky: Bullock Pen in Crittenden, KY, and The Real McCoy in Burlington, KY. (Fig. 1). Chemical parameters were obtained using a YSI SONDE. Lake depth (m) and Secchi depth (m) were recorded using a Secchi disc. Plankton samples were collected at the shore and in open water vertically and horizontally using a 64 µm plankton net. Fifty ml samples of each tow were added to a sterile centrifuge tube. To preserve the samples, one drop of Lugol’s iodine was added to each 50 ml sample. Half of an Alka-Seltzer tablet was added to each sample to allow the zooplankton to expand, making them easier to identify under a microscope. The preserved samples were mixed, and a 1 ml sample was taken from each using a disposable pipette. The 1 ml sample was placed around the edges of a petri dish, and analyzed through a dissecting microscope. We counted and recorded the amount and types of zooplankton, and types of phytoplankton in each sample. Toxin levels from cyanobacteria found in The Real McCoy were determined using an ELISA (Enzyme Linked Immunosorbent Assay) assay.
We identified 4 genera of cyanobacteria in The Real McCoy: Woronichinia, Microcystis, Aphanizomenon and Anabaena. No cyanobacteria were observed in Bullock Pen. There were 10 types of zooplankton covering 9 genera observed between the two lakes. Zooplankton richness and diversity were significantly higher in Bullock Pen compared to that of The Real McCoy. Not only did Bullock Pen have a larger representation of genera than The Real McCoy, the population of each genus was significantly higher. For example, in comparing the horizontal shore tow of the lakes, no zooplankton was present in The Real McCoy. Conversely, 5 genera were found in Bullock Pen (Table 1). From the vertical shore tow, only one Cyclopoid copepod was present in the Real McCoy, while Bullock Pen had 8 genera with significantly larger populations (Table 2). The rotifer Keratella and a single Nauplius larva were observed in the open water horizontal tow from The Real McCoy, while 7 genera were observed in Bullock Pen (Table 3). Daphnia, Keratella and Nauplius larvae were present in low populations in The Real McCoy open water vertical tow. In contrast, 7 genera and Nauplius larvae were present and in much higher populations in the open water tow from Bullock Pen (Table 4). Toxin concentrations from the cyanobacteria in The Real McCoy were 0.5 ppm in the horizontal shore tow and 0.2 ppm in the horizontal whole water tow.
The proliferation of HAB’s in lake ecosystems due to increasing anthropogenic land use are a real threat to the health and sustainability of the planet’s aquatic ecosystems (Paerl and Huisman 2009). Because the toxins that certain cyanobacteria produce have been shown to reduce grazing activity in the cladoceran Daphnia magna and to also inhibit the protein phosphatase activity of other cladocerans Daphnia pulex, Daphnia pulicaria and the crustacean Diaptomus birgei (Bownik 2013), organisms of higher trophic levels in the aquatic food web that feed on these lower-level invertebrates stand to be negatively affected in regards to survival. Therefore, it is imperative to continue to monitor aquatic ecosystems where HAB’s are present to ensure the sustainability of zooplankton species diversity, which in turn will secure the sustainability of the aquatic food chain.
Full study presented at the Scientific Symposium of the Ohio River Basin Consortium for Research and Education, September 2014.
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- Bownik, Adam. 2013. Effects of cyanobacterial toxins, microcystins on freshwater invertebrates. Pol. Natur. Sc. 2892: 185-195.
- Christoffersen, K. 1996. Ecological implications of cyanobacterial toxins in aquatic food webs. Phycologia: 35,6S: 42-50.
- Paerl HW, Fulton, RS III, Moisander, Pia H, Dyble, Julianne. 2001. Harmful freshwater algal blooms, with an emphasis on cyanobacteria. The Scientific World 1: 76-113.
- Paerl, HW, Huisman, J. 2009. Climate change: a catalyst for global expansion of harmful cyanobacterial blooms. Environmental Microbiology Reports 1: 27-37.
Featured Image: Kentucky’s Barren River Lake. (Credit: Wikipedia User Bedford via Wikimedia Commons)