Management and rehabilitation of aquatic ecosystems: introduction and synthesis

Healthy freshwater and estuarine ecosystems are vital to human health, well-being and culture (Harmsworth et al. 2011; Schallenberg et al. 2013; Wood et al. 2016). However, these ecosystems are under stress internationally from a range of pressures, including land use intensification, urbanisation, dam building, bioinvasions and climate change, resulting in high biodiversity loss and widespread eutrophication (Kennish 2002; Dudgeon et al. 2006; Vorosmarty et al. 2010; Larned et al. 2016). Water-related issues are the New Zealand public’s top environmental concern (Hughey et al. 2013). A range of policies and management practices are being applied in different parts of the world to address these issues (e.g. Moss 2004; McDowell et al. 2016) but all struggle to deal with the cumulative, multiple-stressor and legacy effects of contaminants and habitat degradation. Therefore, significant knowledge gaps and uncertainties remain. This special issue presents findings from two recently completed New Zealand research programmes on: (1) the cumulative effects of stressors and (2) the rehabilitation of freshwaters and estuaries. The studies included aimed to improve management, to prevent further deterioration and to rehabilitate degraded systems.

The primary legislative tool for management of freshwater and estuaries in New Zealand is the Resource Management Act 1991 (RMA) which takes an ‘effects-based’ approach to controlling activities which have the potential to degrade aquatic values (c.f. ‘inputs-based’ approach favoured by the European Union). However, much of the decision-making in the early plans under the RMA happened through case-by-case assessments of environmental effects of individual proposed activities, and it has only been in the last decade that it has begun to be applied to manage cumulative effects (Rouse & Norton 2017). Rouse and Norton (2017) describe how the influential, collaborative process called the Land and Water Forum (LAWF, representing 60 stakeholder groups) has contributed to a major shift in how freshwater planning is done in New Zealand and has therefore altered the roles of scientists operating near the science-policy interface. LAWF’s early report underpinned the New Zealand Government’s National Policy Statement for Freshwater Management (NPS-FM (MFE 2011) and amended in 2014 (MFE 2014)) that promotes the use of collaborative processes to plan development and includes the mandatory requirement for councils to set objectives and associated limits on resource use, both for water quantity and quality. Rouse and Norton (this issue) argue that these changes have emphasised the importance of scientists as ‘honest brokers’ of multidisciplinary information for those involved in collaborative policy-making processes, in particular by describing consequences of alternative scenarios.to inform the setting of capacity-based limits.

As the indigenous people and Treaty of Waitangi partners, Māori have particular rights and strong interests on the governance and management of freshwater bodies (Harmsworth et al. 2016). Recent legislative and policy reforms have enhanced Māori involvement in decision-making through co-governance arrangements, joint management agreements, statutory acknowledgements and recognition of ownership (Hudson et al. 2016). Environmental report cards provide a tool that can enhance cross-cultural understanding and alignment of values to assist co-management of water by Māori and Regional Councils. Tipa et al. (2017) discuss the value of environmental report cards as an integrative tool to support restoration strategies and collaborative processes by condensing diverse monitoring information about the health and well-being of catchments into a relatively simple and familiar format to increase understanding. They present two case studies from the Waikato and Waitaki River catchments in New Zealand where report cards based on eco-cultural frameworks have been developed that explicitly link indicators of the state or condition of the catchment with the vision and goals articulated by iwi/hapū.

Reference benchmarks are needed to assess the contemporary status of rivers and to establish restoration targets. Clapcott et al. (2017) developed predictive models to use available geographic information to estimate site-specific reference values for the macroinvertebrate community index (MCI), which is used widely in New Zealand to indicate a range of human impacts on wadeable streams. They found that using fitted flexible models (boosted regression tree and random forest models) and resetting predictors to reflect natural state provided accurate predictions of reference condition MCI. The proportion of native vegetation cover in upstream catchments was the primary predictor of MCI in all models, while secondary predictors varied regionally.

Community-based monitoring of freshwaters and estuaries (a form of citizen science) is an area of growing interest globally that provides opportunities to augment professional monitoring and expand the information base for policy development and management, if the reliability of volunteer assessments is verified. Storey & Wright-Stow (2017) compared volunteer benthic macroinvertebrate data with professional data from parallel monitoring over 18 months at nine sites across New Zealand. They found strong correlations and little bias between volunteer data using volunteer indices and professional data using a professional index and conclude that, with suitable training and support, volunteers can reliably assess stream ecological condition, with only slightly higher variability than professionals.

Schallenberg et al. (2017) provide an example of the application of science within the context of the NPS-FM to recommend nitrogen and phosphorus loading limits to avoid a regime change via loss of keystone Ruppia macrophyte communities in Waituna Lagoon, an intermittently closed and open lake/lagoon system (ICOLL). They used three independent lines of evidence (international literature review, expert assessment based on Australasian data and deterministic coupled hydrodynamic-ecological modelling) to evaluate the likely threshold/tipping point for Ruppia collapse. The three lines of evidence helped the Waituna Lagoon technical advisory group to reach a consensus on nitrogen and phosphorus load limits (and required load reductions) to safeguard the Ruppia beds. The authors reviewed the challenges in applying scientific knowledge to lake and ICOLL management and highlight the need to apply the precautionary principle and adaptive management to ecosystems that are susceptible to rapid regime shifts where there is uncertainty in the scientific evidence.

Another keystone macrophyte, the seagrass Zostera muelleri is addressed by Matheson et al. (2017), but from the perspective of restoration of estuarine habitat once the conditions that caused seagrass collapse have been rectified sufficiently. They found that two of the three transplant methods they tested were equally effective and demonstrate that donor sites recover within nine months.

Unlike the declines commonly recorded for Ruppia and Zostera communities, native intertidal mangrove (Avicennia marina) forests in New Zealand’s northern estuaries are often the focus of management to remove them or control expansion. This is motivated by public perception that expansion of mangroves reduces amenity values associated with former sand and mudflat habitat, such as unobscured water views and ready boat access. Bulmer et al. (2017) assessed a range of mangrove clearance and removal methods designed to restore historically present sandflats while minimising or avoiding adverse impacts. They found that sites cleared by hand were associated with lower disturbance to the sediment than mechanical clearances, and sites exposed to higher hydrodynamic forces showed greater signs of transition towards adjacent sandflats. However, sediment characteristics and macrofaunal communities from clearance sites remained more similar to intact mangrove than adjacent sandflat over the 36-month sampling period suggesting that transition to sandflat conditions is unlikely to occur in the first five years following clearance. These findings regarding the consequences of current mangrove management options will provide valuable insight for future mangrove clearance operations and monitoring, temper expectations of likely clearance outcomes (i.e. that sites will transition to sandflat) and inform the development of future management approaches.

Achieving NPS-FM bottom-lines and locally assigned targets for stream water quality and ecological health in agricultural catchments is likely to require a range of on-farm and riparian mitigations (McDowell et al. 2013). However, there are few case studies on stream responses to mitigation implementation to guide planning and expectation of response times. To address this Wright-Stow & Wilcock (2017) describe the changes in stream water quality attributes during progressive adoption over 7–13 years of mitigations in five contrasting geographic settings dominated by dairy farming, across New Zealand. The mitigations focused on livestock exclusion from streams and dairy effluent management. They found a decrease in suspended solids concentrations for all streams, generally increasing water clarity, and lower E. coli concentrations in two streams. Responses of macroinvertebrate metrics of stream ecological health were more mixed and suggest these may be constrained by lack of connectivity with source populations of sensitive species, as well as habitat quality, time and/or mitigation practice effectiveness.

The issue of macroinvertebrate recolonisation of streams where habitat and water quality have been enhanced is explored in the papers of Graham et al. (2017) and Storey et al. (2017). The ‘natural experiment’ provided by the rewetting of intermittently dry headwater sections of streams upstream of permanently flowing reaches is used by Graham et al. (2017) to determine how far upstream aquatic insects disperse by crawling (as larvae) to colonise rewetted habitat, or by flying when the intermittent stream reaches are dry. They conclude that benthic crawling is unlikely to contribute a large number of colonists to a newly restored site if it is further than 200 m upstream of a source population. Flight by adult insects appears to have much greater recolonisation potential, with many caddisfly species regularly found up to 1500 m from the nearest stream source. This information will guide the spatial arrangement of restoration efforts aimed at improving ecological health/biodiversity of streams.

Storey et al. (2017) address the reach-scale influences on oviposition of a range of stream invertebrate taxa (caddisflies, blackflies and a limpet) and discuss how local habitat features may enhance restoration of diverse communities. They found that hydrobiosid and hydropsychid caddisflies, two species of Austrosimulium blackflies and the limpet Latia are all selective to some extent in their oviposition behaviour, generally favouring rocks that are large, emergent or reaching near to the water surface and not embedded in fine sediment. Therefore, they recommend that to promote recolonisation and/or persistence of these taxa, restored stream reaches need some rocks (or other features) with these characteristics.

This collection of papers fills key knowledge gaps with regard to the effective management and rehabilitation of degraded freshwater and estuarine environments. We hope these advances in understanding will assist communities and agencies in effective application of the NPS-FM (MFE 2014) and support improved freshwater and estuary management and restoration outcomes.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research was funded by the Ministry of Business, Innovation and Employment Programmes Maintenance and Rehabilitation of Aquatic Ecosystems, Contract no. C01X1002 and Management of the Cumulative Effects of Stressors on Aquatic Environments (Contract no. C01X1005).