Upper Midwest Environmental Sciences Center
Potamogeton crispus: Detection in LTRM summer surveys, seasonal biomass and nutrient standing stocks, and links to water quality in Pools 7 and 8 of the Upper Mississippi River System
Drake, D., J. Kalas, and S. Giblin2017. Potamogeton crispus: Detection in LTRM summer surveys, seasonal biomass and nutrient standing stocks, and links to water quality in Pools 7 and 8 of the Upper Mississippi River System. A completion report submitted to the U.S. Army Corps of Engineers’ Upper Mississippi River Restoration Program from the U.S. Geological Survey, LTRM-2016PC2. 33 pages with 4 appendixes.
- Native aquatic vegetation in the Upper Mississippi River typically reaches maximum biomass and flowers in the summer, senesces in the fall, and remains largely dormant throughout the winter. In contrast, invasivePotamogeton crispus undergoes a conspicuous mid-summer senescence, fall germination, and grows and photosynthesizes through the late fall, winter, and early spring. It reaches maximum biomass and flowers in early-to mid- May. Thus its role in off-season gas exchange and nutrient cycling may be substantial. In an effort to understand biological shifts in primary production and water quality associated with P. crispus, we quantified its seasonal biomass, under-detection in LTRM (summer) surveys, May N and P standing stocks, and we compared seasonal water quality within established beds of P. crispus to those within a reference area.
- The average standing biomass of P. crispus in May was very high in two areas supporting beds of the invasive species (2.65 and 1.39 kg FW m-2, respectively) and very low (0.01 kg m-2) in a reference area supporting mostly native aquatic vegetation. In July, 2016 the fresh biomass of P. crispus, including turions, was 0.3% and 1.5% of, and 0% of the May values, respectively. Thus the “sampleable” biomass of P. crispus was approximately 100x higher in May than it was in mid-July (the mid-point of the LTRM sampling season).
- 2016 LTRM aquatic vegetation surveys underestimated the maximum (May) prevalence of P. crispus in three study areas by 0%, 40% and 100%. Underestimation was more severe where the May biomass of P. crispus was low. This study produced a rough, pool-wide estimate of 60-80% underestimation of P. crispus percent frequency occurrence in Pool 8, excluding the few areas where it is very abundant. Thus LTRM records of percent frequency occurrence should be multiplied by a factor of ~3 to reflect the true (May) prevalence of P. crispus. This correction factor is rough, but will be refined with limited, additional sampling in May 2017 and 2018. A similar correction factor likely applies to LTRM vegetation surveys in pools 13 and 4, but relationships may be influenced by latitudinal differences in climate or other large-scale drivers and should be validated and adjusted prior to application.
- Foliar nutrient content in P. crispus and the two most common native submersed aquatic species, Certaophyllum demersum and Elodea anadensis, were in the ranges of 2.5 – 4% N, and 0.4 – 0.56% P. These concentrations are higher than most published values for freshwater angiosperms. In the study area with the highest abundance of P. crispus (Stoddard Bay) the estimated May standing stocks of N and P were ~9,570 kg and ~1,260 kg, respectively. Nutrients are mineralized during senescence in June and July and may contribute significantly to local nutrient budgets. Scaling up nutrient dynamics to the ecosystem level with reasonable confidence will require further development of the correction factor and maximum biomass estimates.
- In 2016, dissolved oxygen concentrations (DO) documented in P. crispus beds relative to a reference area were higher in January, similar in May, varied considerably between the three areas in July, and were similar in October. Relative to the reference area, diurnal fluctuation was stronger in the P. crispus–dominated areas in May and October. Both January and July patterns in DO may have been influenced by unusually high water and atypical patterns in hydrology in 2016. There were no clear patterns in nutrient concentrations, chlorophyll a, or phytoplankton assemblages at the P. crispus areas vs. the reference area. Water quality within the specific areas appears to be more strongly driven by mass movement and exchange of water than by local aquatic vegetation dynamics.