Stream macrophytes and ecosystem health
The types and amounts of macrophytes in streams has implications for ecosystem health.
Benefits of macrophytes in streams
Macrophytes in streams at low or intermediate levels provide beneficial ecosystem services. As primary producers they are:
- a substrate for biofilms
- food for invertebrate growth
- a heterogeneous habitat for invertebrates and fish
- reduce the downstream transport of fine sediment,
- and they intercept and assimilate nutrients.
Negative impacts of macrophytes
However, macrophytes can also form excessive or nuisance growths that impede water flow, increase flooding risk, reduce habitat diversity for instream fauna in general, reduce 'fishability' and cause large daily fluctuations in dissolved oxygen (DO) and pH.
Invasive submerged plants are described as transformer species because of the degree they alter the local environment, with excessive growths of these plants identified as a significant environmental issue in some regions of New Zealand.
Measuring macrophytes to meet National Policy Statement for Freshwater Management objectives
The National Policy Statement for Freshwater Management (NPSFM, 2014) and the National Objective Framework (NOF) has prompted regional councils to seek to define and measure levels of macrophytes in streams that indicate a decline in ecosystem health compared with a healthy or reference state. Using the NOF framework of ‘excellent’, ‘good’, ‘fair’ and ‘poor’ states for an attribute, knowledge and data on stream macrophytes can be used to develop a macrophyte attribute table for specific regions.
Working with Auckland Council staff a NIWA research team has developed a draft stream macrophyte attribute table for Auckland. The next step is to obtain further field data to refine the attribute table.
Current approaches for managing macrophyte abundance and opportunities for research
Management of macrophyte abundance to improve ecosystem health can be informed by the threshold between desired states within the table. Riparian shading is frequently advocated as a means to improve stream habitat for fauna. However the level of light reduction that is required to achieve or maintain a desired level of macrophytes, such as minimising the development of weed beds and providing some habitat heterogeneity is less well defined. This is of particular importance for transformer species that are the usual culprits of excessive weeds growth, yet are well recognised for their environmental tolerances.
In addition, it is important to note that shading is just one factor that influences light availability to submerged macrophytes, for which light attenuation through the water column is also relevant. Future research seeks to address this knowledge gap.
Mary de Winton
Wolfgang Kanz, Auckland Council
Matt Bloxham, Auckland Council