Consensus Report

Each report is produced by a committee of experts selected by the Academy to address a particular statement of task and is subject to a rigorous, independent peer review; while the reports represent views of the committee, they also are endorsed by the Academy. Learn more on our expert consensus reports.

Key Messages

  • It will be important for the District to clearly define the actual distribution and frequency of Phase II sampling,
  • Additional water withdrawals may increase the likelihood, duration, and areal extent of water column stratification and bottom water hypoxia in the lower St. Johns River under low-flow conditions, which was not discussed in the Phase I report.
  • Amphibians, reptiles, invertebrates, and plants have proven to be valuable indicators of hydrologic impacts to wetlands in monitoring programs and might be considered as indicators in Phase II.
  • Based on the modeling results, the change in the average discharge of groundwater following water withdrawal was not predicted to be particularly significant in the middle and upper basins, although the change in chloride flux was predicted to be significant.
  • During Phase I, the District predicted that projected future water withdrawals could have dramatic consequences on SAV in some areas, especially where V. americana populations now fluctuate in the lower St. Johns River. Although V. americana presumably could migrate further upstream, there is less shallow water area there, so a net loss of habitat is still expected.
  • Experimental studies should be done at as large a spatial scale as possible to avoid artifacts caused by trying to extrapolate results from small sample sizes and small containers to the ambient environment.
  • Improved hydrodynamic and hydrologic modeling during Phase II is expected to provide more spatially explicit predictions of the salinity increases in the littoral zone.
  • It will be important for the District to assemble basic water and nutrient budget information for the St. Johns River basin and its major subunits.
  • More careful analysis in the form of advanced hydrologic and wetland modeling is needed to determine the area to be dewatered as a result of lower water levels, including the timing and duration of dewatering events.
  • Nutrient budgets are needed to conduct a credible analysis of the impacts of water withdrawals on algal blooms in the river and especially in the large lakes that constitute major parts of middle portion of the St. Johns River
  • SAV is a focus of the WSIS because the proposed surface water withdrawals are likely to exacerbate salinity intrusions in the estuarine portion of the river, which could have detrimental effects on local SAV populations.
  • The District should carefully study potential impacts to all fishes in the middle and upper St. John River but particularly those that require shallow areas for spawning and foraging, including but not limited to the centrarchids.
  • The District should consider broadening the range of taxonomic groups used in monitoring wetland impacts.
  • The Phase I report does not adequately address the type or frequency of additional water quality and biological monitoring data needed to adequately assess the impacts of water withdrawals on TMDLs and plankton.
  • The biogeochemistry workgroup identified seven potential effects of additional water withdrawals on biogeochemical processes, all related to the possibility that soil accretion will be reduced and/or oxidation of organic soils will be enhanced in the extensive floodplains of the St. Johns River as a consequence of changes in river stage induced by additional water withdrawals.
  • The committee supports the District's commitment to study the effects of water withdrawal on benthic invertebrate communities.
  • The district has proposed investigating additional species in the Phase II work plans; however, there is no detail or methodology provided on how the different assemblages presented (e.g., reptiles, amphibians) might be used to indicate impacts.
  • The effects of high concentrations of CDOM (that may result from water withdrawals) on phytoplankton ecophysiology were not explored during Phase I, although the biogeochemistry workgroup did consider increased concentrations of CDOM.
  • The fish data analyses need to be integrated with other components of the regional biota (e.g., benthos, decapods, SAV) that also may be influenced by salinity shifts.
  • The hydrodynamic modelers should have a documented process for determining scenarios and data needs for the other six workgroups.
  • The primary goal of the Phase I groundwater modeling was to predict whether discharges of groundwater into the St. Johns River would change if the river stage dropped due to water withdrawals.
  • Three of the potential effects were considered to have potentially high significance: (1) reduced nutrient sequestration, (2) increased release of colored dissolved organic matter (CDOM), and (3) increased production and reduced sequestration of greenhouse gases produced within inundated organic soils.