Assessment Comments

Assessment is based on results of (1) ISU lake survey from 2000-04, (2) ISU reports on lake plankton communities from 2000-05, and (3) surveys by IDNR Fisheries Bureau.

Basis for Assessment

SUMMARY:  The Class A (primary contact recreation) uses are assessed (monitored) as "fully supporting / threatened (declining trend; impaired)" due to an ongoing increase in chlorophyll concentrations and declines in water transparency that suggest the potential for aesthetically objectionable conditions at this lake.   In addition, relatively frequent violations of the Iowa criterion for pH also suggests impairment of the Class A uses.   The Class B(LW) aquatic life uses are assessed (monitored) as "partially supporting" due to the relatively frequent violations of Iowa's Class B(LW) pH criterion.   In addition, siltation impacts at this lake remain a concern.   Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.   The sources of data for this assessment include (1) results of the statewide survey of Iowa lakes conducted from 2000 through 2004 by Iowa State University (ISU), (2) information from the IDNR Fisheries Bureau, and (3) information on plankton communities collected from 2000 through 2005 for the ISU lakes survey.  

EXPLANATION:  Results from the ISU statewide survey of Iowa lakes suggest that the Class A uses of Volga Lake are currently "fully supported”, but adverse trends in levels of chlorophyll-a and water transparency threaten the continued "full support" of these uses.   Using the median values from this survey from 2000 through 2004 (approximately 15 samples), Carlson's (1977) trophic state indices for total phosphorus, chlorophyll-a, and secchi depth are 67, 61, and 60, respectively.   According to Carlson (1977), the index value for total phosphorus places this lake in the middle range between eutrophic and hyper-eutrophic lakes; the index value for chlorophyll-a is at the lower boundary between eutrophic and hyper-eutrophic lakes; and the index value for secchi depth is at the upper boundary of eutrophic lakes.   These index values suggest that, despite the relatively high levels of phosphorus, the production of suspended algae is somewhat elevated and water transparency relatively good.   These results suggest that this lake does not have impairments due either to aesthetically objectionable blooms of algae or due to poor water transparency.  

A comparison of annual TSI values for the 2000-2005 period at Volga Lake, however, suggests worsening trends in levels of chlorophyll-a and Secchi depth.   Based on ISU lake survey monitoring data, annual summer TSI values for both parameters have generally worsened over the five-year period.   TSIs for chlorophyll-a were low (exceptional) in 2000 (49) and 2001 (50) (approximately 7 ug/l) but increased (worsened) from 2002 through 2004 (68, 64, and 70, respectively) (roughly 50 ug/l).   Similarly, TSIs for Secchi depth were low (exceptional) in 2000 (54; 1.5 meters) and 2001 (40; 4 meters) but increased (worsened) from 2002 through 2004 (62 (0.9 meters), 75 (0.3 meters), and 63 (0.8 meters), respectively).   While the overall (2000-04) TSI values for chlorophyll-a and Secchi depth are less than 65 and thus remain in the “fully supported” range, a continuation of these adverse trends will likely result in impairment and addition of this lake to Iowa’s Section 303(d) list.  

According to Carlson (1991), the occurrence of a high TSI value for total phosphorus with a relatively low values for chlorophyll-a indicates that some factor (e.g., nitrogen limitation, zooplankton grazing, or some other factor) other than phosphorus limits production of algae.   The results of ISU monitoring suggest the somewhat elevated levels of inorganic turbidity at this lake may function as the primary non-phosphorus limitation on algal production at this lake.   Neither nitrogen limitation nor zooplankton grazing appear to influence algal production.   In terms of all Iowa lakes sampled, data from the ISU survey show relatively small populations of zooplankton species at this lake that graze on algae.   Sampling from 2000 through 2005 showed that Cladoceran taxa (e.g., Daphnia) comprised about 30% of the dry mass of the zooplankton community of this lake.   The average per summer sample mass of Cladoceran taxa over the 2000-2005 period (40 mg/l) was the 20th lowest of the 131 lakes sampled.   These results suggest little if any non-phosphorus limitation due to zooplankton grazing at Volga Lake.  

Based on median values from ISU sampling from 2000 through 2004, the ratio of total nitrogen to total phosphorus for Volga Lake is 45.   This ratio is relatively high and does not suggest any limitations to algal production due to nitrogen availability.   This relatively high ratio does suggest an excess of total nitrogen at this lake (Volga Lake had the 18th highest 2000-04 median total nitrogen concentration of the 131 lakes sampled:  3.5 mg/l).  

The levels of inorganic suspended solids (ISS) at this lake are low but appear to be increasing over time.   The median level of inorganic suspended solids in the 131 lakes sampled for the ISU lake survey from 2000 through 2004 was 5.2 mg/l; the median level at Volga Lake for this period was 3.8 mg/l.   This median level is the 49th lowest of the 131 lakes sampled.   A comparison of the median ISS value for the current (2006) assessment (3.8 mg/l) reflects a worsening over that of the previous (2004) assessment period (1.2 mg/l).   The median ISS value of the four samples collected at this lake during for the 2000-2001 period was 1.9 mg/l; the median for the eight samples collected at this lake during 2002-2004 period was 5.1 mg/l.  

Thus, the water quality conditions at this lake suggest that the Class A uses should be assessed as “fully supported / threatened (impaired)” due to adverse trends in levels of chlorophyll-a, Secchi depth, and inorganic suspended solids.  

The levels of nuisance (=noxious) algal species (i.e., bluegreen algae) at this lake do not suggest an impairment of Class A uses.   While data from the ISU survey from 2000 through 2004 suggest that bluegreen algae (Cyanophyta) comprise a significant portion (73%) of this lake’s summertime phytoplankton community, sampling from 2000 through 2004 showed that the median per summer sample mass of bluegreen algae at Volga Lake (12.5 mg/l) was the 69th lowest of the 131 lakes sampled.   This level is approximately equal to the median level for the 131 Iowa lakes sampled (12.2 mg/).   The presence of a relatively small population of bluegreen algae at this lake does not suggest a potential violation of Iowa’s narrative water quality standard protecting against occurrence of nuisance aquatic life.   This assessment, however, is based strictly on a distribution of the lake-specific median bluegreen algae values for the 2000-2004 monitoring period.   Median levels less than the 75th percentile of this distribution (~29 mg/l) were arbitrarily considered by IDNR staff to not represent an impairment of the Class A uses of Iowa lakes.   No criteria exist, however, upon which to base a more accurate identification of impairments due to bluegreen algae.   Thus, while the ability to characterize the levels of bluegreen algae at this lake has improved over that of the previous (2004) assessment due to collection of additional data, the assessment category for assessments based on level of bluegreen algae nonetheless remains, of necessity, "evaluated" (indicating an assessment with relatively lower confidence) as opposed to "monitored" (indicating an assessment with relatively higher confidence).  

Information from the IDNR Fisheries Bureau suggests that the Class B(LW) aquatic life uses should be assessed as "fully supported".   Siltation impacts in the lake, however, remain a water quality concern.   The ISU lake survey data, however, suggest impairments of both the Class A and Class B(LW) uses due to high levels of pH at this lake.   Only one violation of the Class B(LW) criteria for dissolved oxygen occurred in the 13 samples collected during summers of 2000 through 2004.   Based on IDNR’s assessment methodology, the occurrence of this violation does not suggest an impairment of aquatic life uses at this lake.   Five of 15 samples, however, violated the Class A,B(LW) criterion for pH (maximum = 9.2; minimum = 8.1 pH units).   Based on IDNR’s assessment methodology, these results suggest that significantly more than 10% of the samples exceed Iowa’s pH criteria and thus suggest an impairment of the Class A and Class B(LW) uses of this lake.   These violations, however, likely reflect primary productivity at Volga Lake and do not reflect the input of pollutants into this lake.

Fish consumption uses remain "not assessed" due to the lack of fish contaminant monitoring at this lake.