Mette Schou Frandsen:
Ecohydrological investigations of a groundwater-lake system

Date: 31-10-2014    Supervisor: Ole Pedersen

This PhD project is a cross-disciplinary study combining hydrological and biological methodology to better describes the lake-catchment interaction seen in an ecological perspective.

The topics investigated were:

  • Does groundwater re- and discharge affect the growth of submerged vegetation? (Paper I).
  • Does dense bottom vegetation affect the small scale hydrology of the lake bed sediment? (Paper 2).
  • How can natural tracers (δ 18O) be used to quantify the temporal variation in groundwater seepage dynamics? (Paper 3).
  • Is it possible to combine ecological data of surface water chemistry and data on groundwater chemistry to stoichiometrically describe changes in the lake in a historical time frame? (Paper 4).

The main conclusions from the study are:
  • When evaluating the ecology of a groundwater-lake system, both hydrological and biological parameters are needed to accurately describe the factors affecting the system.
  • The biology and ecology of the lake (i.e. submerged vegetation and surface water chemistry) are highly affected by groundwater seepage.
  • The hydrology at the surface-water-interface is highly affected by the biology (i.e. submerged vegetation).
  • Groundwater-lake systems are very dynamic systems on a spatial scale. Variability in meteorology can lead to variability in the hydrology, and in some cases ignite transient effects that are temporally distinct and difficult to capture.
  • To some extend the lakes acts as sentinel for all the in and out-puts to the system as well as the in-lake processes. By combining this ecological view with hydrology, it is possible to gain information on the historical development in the surface water chemistry.

Lake Hampen is a Danish flow through lake receiving almost 2/3 of its water through groundwater discharge. In this setting I investigated the interrelationship between hydrology and biology.

I found that groundwater seepage significantly affected the growth rates of submerged isoetids (small rosette type plants) by providing them with a continuous supply of nutrients and inorganic carbon. The seepage rates were strongly correlated to the growth responses and the plant mass was higher in treatments where the plants were subjected to groundwater seepage compared to treatments with no groundwater seepage.

I also found that the submerged vegetation conversely had a significant effect on the small scale hydrology of the lake bed sediment. On densely vegetated areas (~9000 plants m-2), the vertical hydraulic conductivity was lower compared to non-vegetated sediment. Disturbing the top layer of the sediment lead to a significant increase in hydraulic conductivity on the vegetated sediment, whereas the non-vegetated sediment was not affected by this. The reasons for the lowered hydraulic conductivity seems to be an combination of the organic content in the sediment (i.e. the roots of the plants) and a vegetation induced entrapment of fine particles in the sediment.

Over the course of three years I followed the small scale variation in the natural tracer, δ18O, and nitrate in the main discharging area of the lake to follow an ongoing flow reversal in the system. By tracking the groundwater – lake water signal using only the distribution of δ18O it was clear that lake water had penetrated the lake bed sediment down to at least 1.25 m during the flow reversal. This was also clear looking at the nitrate data and during the flow reversal the nitrate concentrations in the sediment was significantly lower than under normal flow conditions. All the nitrate was denitrified before reaching the lake and the estimated denitrification rates were lower than the assumed capacity.

In Lake Hampen, the alkalinity suddenly started to increase during the mid-1970s. Using a simple four step modeling approach, I found that denitrification of nitrate discharging to the lake, stoichiometrically could explain the development in the alkalinity in the surface water. This method gave a surprisingly accurate picture of the yearly development in surface water alkalinity despite the somewhat simplified approach used to estimate the historical input of nitrate with the groundwater.

In conclusion I strongly encompass the notion that a cross-disciplinary approach greatly qualifies the results of ecological studies.