Lake Tahoe Nearshore Evaluation and Monitoring Framework- Phytoplankton

No Project associated with this Finding

Finding Details

The free-floating algae in lakes are referred to as phytoplankton. These organisms typically form the base of the aquatic food web as they use sunlight, carbon dioxide and nutrients to create organic biomass. In a simple food chain, these organisms are consumed by zooplankton, and in turn by higher order invertebrates and fish. Phytoplankton can also be part of the microbial food loop which includes dissolved organic carbon, bacteria and the entire microbial food web, protozoans and other microzooplankton. When present in too high a level phytoplankton degrade water quality and drive cultural eutrophication.

Phytoplankton consists of diverse assemblage of many different major taxonomic groups, including, but not limited to diatoms, green algae, cryptophytes, chyrsophytes, dinoflagellates, euglenoids and blue-green algae (cyanobacteria). These groups, and the individual species with each group, have different pigments, morphological characteristics, resource requirements, growth rates and sinking velocities (e.g. Reynolds 2006). Their size can range over several several orders of magnitude (~0.2-200 μm).

Hutchinson (1961) raised the issue of what he called the “paradox of the plankton”. This refers to the fact that many tens of phytoplankton species can coexist in lake water. A foundation of ecological competition theory holds that if two organisms compete for resource one will win out over the other. If so, Hutchinson postulated that phytoplankton were able to achieve niche separation based on naturally occurring gradients of light, nutrient and water movement; differential predation; combinations of all or some of these factors; and an otherwise constantly changing environment. This is important as it explains why so many species are present, and why species change as trophic status or other conditions change. This has allowed scientists to classify phytoplankton species composition on the basis of trophic state and other lake characteristics.

As lake conditions change over the course of a year, the phytoplankton community will experience seasonal succession (EPA 1988). This phenomenon will generally repeat itself between years provided there are no major environmental changes. These seasonal differences are a natural occurrence and are not particularly useful as indicators of water quality or changing trophic status. However, based on numerous, world-wide observational studies of lake phytoplankton some general conclusions can be made with regard to species composition and trophic status (e.g., Eloranta 1986, Wetzel 1983, Reynolds 2006, Hunter TERC unpub. data).

In general, ultra-oligotrophic and oligotrophic lakes contain diatoms, chrysophytes and dinoflagellates, with diatom dominance. However, it is important to emphasize that all the individual species that make up these larger taxonomic groups are found in only oligotrophic conditions. Select species in all these groups are found in water across the entire trophic status spectrum. As trophic status moves away from oligtrophy and reaches eutrophy other groups become more prevalent, e.g. cyanobacteria, euglenoids, green algae and different species of diatoms. Species composition is very important in the food web and for the productivity of the grazers and consumers. Diatoms contain relatively large amounts of highly unsaturated fatty acids, a material with very high food quality. Certain species of cyanobacteria, in eutrophic bloom conditions can create nuisance conditions, release toxins and are create taste and odor problems, and are therefore quite undesirable.

A total of ca. 380 algal taxa were recorded in 128 littoral phytoplankton samples during the UC Davis study. Diatoms accounted for 36 percent of the total number of species with approximately three-quarters of these being benthic forms. Besides diatoms, the green algae and chrysophytes were also rich in number contributing 86 and 50 species, respectively (Eloranta and Loeb 1984).

Generally, the major taxonomic groups that dominated littoral zone phytoplankton were found to be similar to those found in the pelagic waters (Loeb, 1983). In particular, this was the case for the major biomass dominants.

Of all the study sites, the south shore stations had the highest species diversity (Loeb 1983). In addition, that study found that three groups which are most indicative of lake water fertility (green algae, cyanophytes and euglenoids) were more abundant at the south shore versus the other stations. SS-3, located 50 m off the western channel of the Tahoe Keys Marina consistently had the highest diversity of phytoplankton.

In comparison, the percent composition of the major taxonomic groups in the pelagic waters from 1982-2010 is shown in Figure 14-1 (TERC 2011). The contribution of chyrsophytes and dinoflagellates was 5-10 and 10 percent higher, respectively, during 1982 in the nearshore versus open water. Cryptophytes were 10-15 percent lower in the nearshore. Despite this differences, the distribution of the major taxonomic groups were very similar between the nearshore and the open water in 1982. While there have been some changes in the percent composition in the open water phytoplankton over the years, the major taxonomic groups and the relative composition remain similar Figure 14-1. For analysis of changes in community composition and individual taxa, samples shouldbe taken a series of 9 sites around the lake corresponding to various levels of watershed development. While more discussion will be needed to finalize these sites, a possible set of stations includes, Rubicon Point, Meeks Bay, Tahoe City, Kings Beach, Glenbrook, Zeyphr Cove, Stateline south, off Tahoe Keys and Kiva Beach. Since the objective is to identify a high abundance of unwanted species, two sampling dates should be selected; both during the summer when public use of the nearshore is maximum.

To determine the species associated with high levels of phytoplankton (to determine if potential bloom-forming organisms are in abundance) samples would be collected and analyzed only when real-time chlorophyll concentrations exceeded a value of ~5 mg/m3 during these perimeter surveys. Based on early sampling results, the chlorophyll value that triggers phytoplankton sampling will be re-evaluated. Sampling would be taken from the same depth as the real-time chlorophyll measurements and collected using the same water pumping system. Phytoplankton samples would be preserved and enumerated according to the methods used by LTIMP for Lake Tahoe water (Winder and Hunter 2008).