Stream health in the Motueka River Catchment -
the role of land use and geology
According to the stream invertebrate communities recently studied at 46
sampling sites throughout the catchment, the majority of the Motueka River
ecosystem is in good health.
Sites in the headwaters are particularly healthy, while there are some
concerns in the lower tributaries. Streams draining ultramafic geology in the
headwaters of the catchment are characterised by low invertebrate densities and
diversity, but the few species that were found are typical of clean mountain
streams. Invertebrate communities in streams draining native forest on the other
geological types in the catchment (Moutere gravel, Separation Point Granite,
hard sedimentary rock, and marble) appeared to be relatively similar to each
other and were also indicative of healthy ecosystems.
The effects of land use were measured by comparisons among pastoral, exotic
forest and native forest sites on Moutere gravel and Separation Point granite
terrains. These comparisons indicated that native forest sites were consistently
healthier than pastoral streams.
Interestingly, exotic forest streams draining Separation Point granite had
healthy invertebrate communities (similar to native forest), while exotic forest
streams draining Moutere Gravels were more degraded and similar to pastoral
streams. To some extent this is the opposite of what we expected given the
concerns about erosion from Separation Point granites. However, significant
amounts of fine clays can be washed into Moutere gravel streams after rainfall
and may affect stream invertebrate communities. Erosion of granite tends to
produce large quantities of coarse sand, but perhaps sand is not as damaging to
invertebrate communities as fine clay.
An alternative explanation for these results relates to low flows at the
study sites prior to our sampling. Separation Point granite acts like a sponge
during rainfall, slowly releasing water and maintaining stream flows even after
long dry periods. In contrast, Moutere gravels store little water and streams
draining these areas of the catchment often dry up. The difference in water
storage capacity between these geologies is particularly evident in areas
covered in exotic forest which are known to reduce water yields even further.
Land use appears to have the major influence on the structure of
macroinvertebrate communities in the Motueka Catchment. However, in some cases
the magnitude of land use impacts can be influenced by the interaction between
land use and geology. Therefore, the geological features of a catchment should
therefore be an important consideration for assessing the impacts of future land
management scenarios on streams and rivers.
A scientific paper describing these results has recently been submitted for
publication to the scientific journal 'Freshwater Biology'.
For further explanation and more details see the poster
(508 kb  ) describing the results
or contact Karen Shearer
or Roger Young at the Cawthron
Institute in Nelson.
Site pictures
Click on the thumbnails to see a larger image.
 |
Stanley Brook Site 44 |
 |
Motueka River (Right Branch) |
 |
Porter's Creek |
 |
Rolling River Site 1 |
 |
Baton River at Ford |
 |
Ellis Stream |
 |
The hard life of a stream ecologist |
| 
