Abstract
As population growth increases, the pressure on natural resources intensifies. A consequence of this is that endemic vegetation is being increasingly converted for other uses. Physically connected ecosystems, such as rivers and estuaries, are directly influenced by upstream and surrounding land use changes. Literature considering the impact of rivers and estuaries as a continuum is limited, despite their physical connection. The Māori ki uta ki tai concept depicting the journey of water, from mountainous regions to the ocean, acknowledges this relationship. However, current management of these systems within Aotearoa (New Zealand) largely neglects to recognise the importance of an integrated approach.
Therefore, this study aimed to investigate the impact of percentage upstream native cover on health indicators (structural and functional) in connected river/estuary systems within the South Otago/Catlins regions of Aotearoa.
This study identified seven catchments within the South Otago/Catlins regions that represented a native land cover gradient. Macroinvertebrates were used an indicator of health in both river and estuary systems. Within rivers, macroinvertebrate indices and percentage upstream native cover were non-linearly related, with the highest scores of both invertebrate metrics occurring within the 40 to 60 percent upstream native cover range. Estuarine macroinvertebrate health improved moderately with increasing upstream native cover. There was no relationship between riverine and estuarine macroinvertebrate health indicated.
Relationships between percentage upstream native cover and functional processes were also investigated. Organic matter decomposition was measured using cotton strips. Cotton strip mass loss in the river systems was weakly negatively related to increasing percentage upstream native cover. Cotton strips incubated in estuaries appeared to have gained weight through sand/salt incorporation, which may have masked any biological loss. Tensile strength loss in rivers revealed no biologically relevant relationship, but tensile strength loss in estuaries showed a weak relationship with percentage upstream native cover.
Organic matter degradation, measured using Rapid Organic Matter Assay (ROMA) plates within estuarine sediments, was moderately negatively related to percentage upstream native cover.
Algal accrual was captured by incubating ceramic tiles. In both rivers and estuaries, algal accrual had a strong, positive linear relationship with increasing upstream native cover.
These findings highlight the existence of relationships between ecosystem health indicators within rivers and estuaries and upstream percentage native land cover. However, this study was unable to demonstrate a clear relationship between the health of the connected rivers and estuaries. The study results emphasise the complexity of considering two connected ecosystems as a continuum. The river and estuary continuum should be further researched so that management can more accurately reflect and protect these two valuable ecosystems.