Interventions & adaptive management
Floating wetlands are distinctive in that they are deployed in a controlled environment for (a) specific purpose(s) (Vymazal., 2010). Ecosystems services that can be facilitated through this ecological intervention include flood control, carbon sequestration, wildlife habitat, growth of biofilms, and entrapment of fine suspended particulates (Vymazal., 2010 & Tanner Et al., 2011). Nutrient removal in floating wetlands is a novel way to interact with eutrophic water bodies that is just as, if not more than, effective as conventional surface-flow wetlands, when under the same conditions (Tanner Et al., 2011).
In the Rotorua case, the primary ecological intervention is the removal of algae (primary negative side effect of algae is the reduction of oxygen in the water body for nutrient removal. Constructed floating wetlands achieve this goal primarily through plant roots descending from the floating mat into the water below, and through the ecological disturbance created by the floating mat itself. The disturbance occurs “by shading the surface, buffering water turbulence, and creating localized anoxic zones beneath the mats” which, as a result, actively settles the suspended algae and soils beneath (Tanner Et al., 2011). More specifically,
“…dissolved oxygen levels typically become depleted beneath the FTWs, due to shading of algal photosynthesis, respiratory oxygen demand of the roots and associated biofilms, and reduced exchange across the water-atmosphere interface. This and the release of exudates is likely to create conditions conducive to microbial denitrification, so enhancing nitrogen removal.” (Tanner Et al., 2011).
The actual plant masses that achieve these goals can be divided into aerial plant tissues, submerged plant tissues, and roots and rhizomes. Aerial plant tissue facilitates subsurface light reduction, reduced wind velocity, nutrient storage, and (particularly important in the Rotorua case due to the integration of the word ‘Rotorua’ as a design feature into the mat) aesthetic appearance. Submerged plant issues provide the microbial and large debris filtering, reduce water current velocity thereby increases sedimentation, excretion of photosynthetic oxygen, and finally, nutrient uptake. The roots and rhizomes prevent vertical flow of debris, further nutrient uptake, and secrete antibiotics for detoxification of root zone which results in pathogen removal (Sundaravadivel, M. and Vigneswaran S., 2001). The primary ecological interventions in Rotorua are sedimentation, denitrification, microbial uptake, plant uptake, water body movement reduction, filtration, and absorption (Sundaravadivel, M. and Vigneswaran S., 2001).
In light of adaptive management, the Rotorua Te Arawa Lakes Programme has a clearly defined governance structure and incorporates all major stakeholders, incorporating multi-level governance, multiple knowledges (governmental, scientific, traditional, and place based local), and financing through multiple streams. The management group, the Rotorua Te Arawa Lakes Programme has a highly effective and responsive management structure due to its composition of governing bodies and their clearly defined roles. As Bobbink, Roland et al., point out the effectiveness and adaptiveness of management structures boils down to community groups engaged in the project; their relationship to one another, and the extent to which all of the individual actors’ activities are synergetic with one another, especially in the face of change.
Rotorua FTW under-going storm damage repair and re-anchoring.
Artistic rendering of pre-FTW abiotic conditions of the lake.