Coeur d’Alene Lake Aquatic Vegetation Survey

PROJECT ID: Baseline Coeur d’Alene Lake Aquatic Vegetation Survey

PROJECT CATEGORY: Aquatic Vegetation Survey

PROJECT LOCATION: Coeur d’Alene Lake (see transect location map)

SITE CHARACTERISTICS: Varies between steep rocky shoreline with little or no aquatic plant growth to shallow, flat-bottomed, mucky substrate areas supporting diverse aquatic plant assemblages.

PROBLEM DESCRIPTION: This project has been designed to address significant data gaps on aquatic vegetation present in Coeur d’Alene Lake. This study is warranted by the following needs:

  • need for biomass and distribution data on which to base harvest plans and effectiveness assessments,
  • need for species-specific nutrient content data to develop estimates of nutrient release (loading) to CDA Lake (this also would effect harvest plans), and
  • need for surveillance to document the presence or absence of invasive, noxious aquatic species such as Eurasian watermilfoil.

PROJECT GOALS & OBJECTIVES: Specific objectives are to perform diver collection of submersed aquatic vegetation species along set transects. All diver-collected samples will be sorted by species and each species sample dried to obtain a standard biomass estimate. Sub-samples of diver-collected plants will be analyzed to determine nutrient (phosphorus and nitrogen) content. Based on the nutrient content data and published literature on species specific nutrient release rates, estimated nutrient loading from submersed plants will be calculated.


  1. Survey transects: The quantitative sampling is a modification of a “line intercept” method where samples are collected along a fixed line which is oriented from a start point on shore by a compass heading. Along this line samples were collected using SCUBA techniques at 3.0 foot depth intervals. At an onshore location, all collected plant materials were sorted by species for drying and weighing by a contract laboratory. Biomass and nutrient content analyses (for phosphorus and nitrogen compounds) were performed on selected samples following standard methods (APHA 1992 or more recent). There were 29 transects sampled during July 2005.
  2. Nutrient Release research: A literature search and communications with university researchers and others was performed to collect available information on nutrient release from aquatic plant species. With the completion of the literature search, aquatic plant species biomass and nutrient data were compiled based on the studied transects. Transect data were expanded to represent a number of “Aquatic Vegetation Regions” in the lake and the area of each of these regions was calculated based on the depths that submersed plants were found along the representative transect(s) for each area. With the area determination (in square meters, m2), a calculation of lake-wide average biomass for each species (in grams dry weight per square meter, g/m2) and lake-wide nutrient content for each of the predominant species (in micrograms phosphorus or nitrogen per gram of dry weight biomass) the lake-wide total nutrient content for each species was calculated. This was then applied to the nutrient release criteria obtained from the literature search and the estimated annual release of phosphorus and nitrogen was calculated.

PROJECT TIMELINE: Project was a one-year effort which began July 2005. The final report was completed April 2006.


The overall conclusion offered from this baseline assessment of submersed aquatic vegetation in Coeur d’Alene Lake is that this growth is healthy, very productive in certain areas (primarily the bays) and moderately diverse. The plants that were identified in the Coeur d’Alene Lake transects were all native species with the exception of Myriophyllum spicatum which was only found in three transects in the southern portion of the lake. Given the extensive growth of this species in the adjacent Lower Lakes area (particularly Chatcolet and Round Lakes), it is expected that this presence in Coeur d’Alene Lake proper will increase significantly in the coming years, absent implementation of control measures.

The two dominant species found, on a biomass basis, were Potamogeon amplifolius and P. robbinsii, both robust pondweed species. On the basis of frequency of occurrence, however, the groups “Elodea species” (primarily E. canadensis) and “Potamogeton species” (the thin-leafed pondweeds) were dominant. Other species which were found to be prevalent included Ceratophyllum demersum, Isoetes sp., P. richardsonii, Ranunculus aquaticus and Sagitaria sp. The macroalgae Chara and Nitella were also seen fairly frequently. Submersed species were found typically at the three to 21-foot depths, with few sites having plant growth to the 24 or 27 foot depth. The highest level of biomass (on a lake-wide basis) was found at the 12 foot depth.

Mean biomass levels (dry weight) of the submersed species, where found, ranged from greater than 60 g/m2 for P. amplifolius and P. robbinsii, to slightly over 1 g/m2 for the smaller species Isoetes and Nyad sp. Maximum sample biomass values approached or exceeded 300 g/m2 for C. demersum, the group “Elodea species”, P. robbinsii and the group “Potamogeton species”. The biomass data compiled by transect (all species and depths) indicated that Cougar Bay had the highest overall density of submersed vegetation, at 68 g/m2. Blue Creek Bay, Echo Bay and the south end of Coeur d’Alene Lake all had average biomass over 40 g/m2. These higher-biomass areas, and in fact most areas supporting submersed plants at all, had soft, organic muck sediments. There were only three of the 29 transects that were found to have no vegetation and these were steeply sloping, open shoreline areas with predominantly rocky substrate.

The assemblages of submersed species were reasonably consistent around the lake. Across various depths there was most often only two or three species or species groups seen. By far the most prevalent assemblage was the group Elodea species with the group Potamogeton species and P. richardsonii. The species most often found alone was P. amplifolius.

Nutrient (phosphorus and nitrogen) analysis data from the seven predominant plant species was used with the biomass data and aquatic vegetation regions to develop an estimate of potential nutrient loading to the lake from the existing plant communities. This estimate could be incorporated into water quality modeling programs to provide additional understanding of the lake’s nutrient budget. The analyses that plant samples from this study were submitted to were total phosphorus, total Kjeldahl nitrogen, nitrate nitrogen and nitrite nitrogen. Of these the total phosphorus and total Kjeldahl nitrogen were the most significant.

Elodea species were found to contain the highest concentration of phosphorus, with a mean of 4,738 µg P/g. Ceratophyllum demersum and Potamogeton species also averaged over 4,000 µg P/g. The lowest mean phosphorus concentration of the plants analyzed was 2,975 µg P/g in P. robinsii. The maximum single total phosphorus concentration reported was 6,050 µg P/g, again in Elodea species, while the lowest single value was 2,180 µg P/g in P. richardsonii.

The highest mean total Kjeldahl nitrogen concentration found was 25,457 µg N/g with the group Potamogeton species and the lowest, 21,000 µg N/g, was with P. amplifolius. The maximum single value found was 27,400 µg N/g with Potamogeton species and the minimum was 13,600 with Elodea species.

The estimation of nutrient release from the aquatic plants in Coeur d’Alene Lake was a key part of this project and literature values were used in this estimate. Phosphorus release from growing plants was assumed to be negligible for all species except Elodea which was applied a rate of 25 µg P per gram of dry biomass per day (over a 180 day growing season). Nitrogen release from growing plants was assumed to be negligible for all species. Phosphorus release from sloughed plant materials (also referred to as “turnover”) was assumed to be 1.25 times the average phosphorus concentration of the plants. Nitrogen release from turnover was 1.0 times the average total Kjeldahl nitrogen concentration. Phosphorus release from senescing plants was 50 % of the average total phosphorus concentration and nitrogen release due to senescence was 10 % of the total Kjeldahl nitrogen concentration.

The nutrient release criteria were applied to calculated total phosphorus and total nitrogen pool values to achieve the estimate of phosphorus and nitrogen release from the plants into the waters of Coeur d’Alene Lake. The nutrient pools were determined from an adjusted total biomass for each species (for which nitrogen or phosphorus was measured) in each of a series of aquatic vegetation regions which were represented by the sampled transects. The resulting total biomass values were multiplied by the average phosphorus and nitrogen contents to achieve the values for the phosphorus and nitrogen pools.

The calculated total annual release of phosphorus from the aquatic vegetation in Coeur d’Alene Lake (excluding the Lower Lakes study area) was 5,603 kg. The calculated total annual release of nitrogen was 18,237 kg. These loadings were found to be consistent with historical loading from a variety of sources, amounting to between 4% and 10% of other phosphorus load sources and between 0.8% and 2% of other nitrogen load sources. Phosphorus and nitrogen loading estimated from the present study were lower than that from the two major tributaries (the Coeur d’Alene and St. Joe Rivers), from wastewater and from precipitation.

Periodic re-surveys should be performed to monitor changes in the submersed plant communities and to help refine the understanding of biomass and nutrient pool distributions. To improve the estimate of nutrient loading to the lake from the aquatic vegetation it would be helpful to survey additional transects, especially in the open shoreline areas.

The results of the Lower Lakes Aquatic Vegetation Survey project, which followed identical protocols to those of the present project, should be combined so that a more complete picture of aquatic plant growth and potential impacts on lake water quality can be considered by the lake managers.