DYRESM-CAEDYM - Waituna Lagoon

The need for increased biodiversity management arose given that the Waituna Lagoon which is a part of the wider Awarua Wetland complex were collectively designated a Ramsar site, signifying a wetland of international importance. The importance of the Lagoon area includes its high biological diversity and a number of endangered species, with Ruppia having been identified as critical for sustaining biodiversity within the lagoon as a “keystone” species.

Recent concerns were raised that increases in the nutrient loads within the catchment could be threatening Ruppia beds in the lagoon and as a result biodiversity in the wider system. This increase in nutrients was thought to be linked to increases in farming intensity in the catchment area in the past 10-15 years with conversion into dairy farming as well as the farming of lower lying peat soils which traditionally were periodically inundated by the lagoon.

In response to these concerns this study aimed to construct and apply process based models in order to assess whether various management scenarios could meet goals for sustaining the natural values of the ecosystem.  The goal of the modelling was to assess suitable catchment and lagoon management techniques to sustain an abundant and stable Ruppia population that would support higher levels of the lagoon’s food web. 

Method

This case study was carried out by the University of Waikato as contracted by Environment Southland. The modelling consisted of two components, the first, a one-dimensional hydrodynamic model (DYRESM) was coupled with the second an aquatic ecological model (CAEDYM).

DYRESM was used to model the vertical distribution of temperature, salinity, density and the vertical mixing processes within lakes and reservoirs.

CAEDYM was used to model varying fluxes which regulate biogeochemical variables (e.g. nutrient species, phytoplankton biomass) including process representations for carbon, nitrogen, phosphorus, dissolved oxygen and inorganic suspended solids.

Findings

The results from this tool are presented as outputs from model simulations of physical, chemical and biological variables within the lagoon. Input data for the model simulations were adjusted to simulate sensitivity of the model to:

• Input data quality and quantity,
• Complexity and formulations used in the model itself,
• Parameters used to adjust physical and biogeochemical environmental responses in the model, and
• Potential management options.

Statistically the model performed well, being able to reproduce the magnitude and dynamics of field measurements, and performed well compared to similar model applications in literature. The model successfully reproduced moderate chlorophyll a variations associated with phytoplankton blooms, and periods with very low biomass (< 3 µg L-1), but did not capture some of the high (> 12 µg L-1) chl a field measurements.

Model statistics suggest the model was most successful at simulating variables such as NO3-N, TN, temperature, salinity, and dissolved oxygen. Where model statistics suggested that the model performed less well, e.g., for PO4-P, NH4-N and chl a, closer examination of field measurements revealed that a high proportion (i.e. 25 – 40%) were below detection limits, which can restrict the ability of model statistics to define model error.

Modelled (DYRESM-CAEDYM) variables (black line = calibration and grey line = validation period) compared with field data (open circles = calibration and filled circles = validation period). A) Total phosphorus (TP; mg/l) and B) total chlorophyll a (chl a; ug/l). Modelled primary producer groups C) phytoplankton  and D) Ruppia and macroalgae

Results from scenarios modelled included for example;

• Removal of microalgae from the model produced simulations in which Ruppia biomass increased.
• A 10% increase in wind speed resulted in a slight increase in chlorophyll a and total suspended solids, a decrease in macroalgae, and increase in Ruppia biomass.
• There is little effect of varying the Ruppia salinity limitation parameter β by ± 10% on Ruppia biomass.
• A 25% reduction in both nutrients (i.e. nitrogen and phosphorus) marginally reduced both chlorophyll a and macroalgae biomass, and increased Ruppia biomass slightly.
• A 90% reduction in both nutrients reduced both macroalgae biomass and chlorophyll a to low levels (i.e. < 1 g C m-2 macroalgae and < 1 µg L-1 chlorophyll a) and more than doubled Ruppia biomass compared to the base scenario.

 Conclusions

The combination of hydrologic and aquatic ecological modelling made this application DYRESM-CAEDYM particularly useful given the context of the coastal setting of this case study. The ability to compare changes to the hydrologic system, such as emptying the Waituna Lagoon at different seasonal times and for differing durations, with changes to both nutrient loading and ecologic activity for comparison meant that management decisions could be best informed on how changes to this system could best preserve Ruppia within the lagoon.

Model simulations using current nutrient loads, suggest that raising the opening trigger water level or reverting to a more “natural” opening regime whereby the lagoon opening is not actively managed, would result in a collapse in the Ruppia beds. Water quality in Waituna Lagoon is known to be greatly affected by opening events, however, model simulations suggest that it is not possible to maintain an abundant and stable Ruppia population in the lagoon with changes to the opening regime alone, i.e., nutrient load reductions are required simultaneously. Comparatively small nutrient load reductions (e.g. 10–25% of current loads) appear to have a limited effect on Ruppia, likely due to the very high level of current loading. Model simulations indicate that to maintain stable and abundant Ruppia under a natural opening regime a substantial reduction in nutrient loads (i.e. 70–90%) is required. However, regular winter openings in combination nutrient load reductions of 50% nitrogen and 25% phosphorus may also achieve this goal.

Associated Models

References

David P. Hamilton, Hannah F. E. Jones, Deniz Özkundakci, Chris McBride, Mathew G. Allan, Joanne Faber & Conrad A. Pilditch (2012): Waituna Lagoon Modelling - Developing quantitative assessments to assist with lagoon management. ERI report number: 004. University of Waikato. Report prepared for Enviroment Southland