Freshwater invertebrates are composed of aquatic insects such as mayflies, caddisflies, stoneflies, dragonflies and true flies (e.g.chironomid midges, blackflies), molluscs such as snails (especially the common mud snail Potamopyrgus) and filter-feeding bivalves (e.g., fingernail clams and freshwater mussels), crustaceans such as freshwater shrimps and amphipods, and worms.  These animals play a vital role in transferring plant based material into animal biomass, which is available to higher predators such as fish and birds.  They also have biodiversity and ecological values, and almost all freshwater invertebrates are native to New Zealand and found nowhere else in the world.

Freshwater invertebrates are influenced by environmental variables such as water velocity, depth, substrate size and the presence of silt, as well as water quality.  They are also controlled by biological interactions with algae, macrophytes, other invertebrates, and fish.  Because of their close link to the stream environment, they are often used as indicators of stream health.  Members of the Mayfly, Stonefly, and Caddisfly orders (or Ephemeroptera, Plecoptera and Trichoptera: EPT) in particular are relatively intolerant to stream degradation, and are often scarce or absent in rivers flowing through highly modified catchments as a result of organic enrichment, sediment inputs, and high temperatures.  Their presence within a river is consequently often used to indicate streams of high “ecological health”.

Urban development has dramatic effects on stream health, reflecting changes to stream flow, loss of terrestrial and bankside vegetation, and reduction in water quality.  Such changes result in streams with low biodiversity values.  Although we do not know what the habitat conditions of the Styx River were like prior to European settlement, we can fairly confidently state that it would have been a relatively fast flowing spring-fed river that flowed over a coarse streambed of cobbles and gravels - similar to what the nearby Otukaikino Stream is currently like.  Water quality would have been high.  The streamside vegetation would have been a mixture of flaxes, tussocks, native shrubs and trees that would have shaded the river in places.  Urban development has resulted in a loss of the native vegetation, and a replacement with non-native grasses, shrubs and deciduous trees.  This would have altered the timing and quantities of organic inputs into the river.  Stream flows are also likely to have been reduced as a result of drainage, and the increased quantities of impervious surfaces within the catchment that would have lowered the amount of water entering the ground.  Sedimentation in the river has increased considerably, and this has been exacerbated by large areas of introduced macrophytes such as Potamogeton crispus and Elodea canadensis that trap fine sediments.  Water quality, although still good at base flow, would undoubtedly decline when it rains as a result of stormwater entering the river, flushing with it a large amount of different contaminants from the surrounding urban areas.

These changes have occurred to a great extent in the Avon and Heathcote rivers, and to a lesser extent in the Styx.  As a result, there have been dramatic changes to the invertebrate faunas of these rivers.  In particular there has been a loss of mayfly and stonefly taxa from the Avon and Heathcote rivers, and a shift in community to tolerant animals such as worms, snails and midges.  The invertebrate communities of these rivers are now dominated by tolerant invertebrates such as worms, snails (Potamopyrgus, Sphaerium and Physa), the amphipod Paracalliope, and a variety of midges.  This fauna is indicative of relatively degraded conditions when compared to non-urban streams.  In contrast, urban development within the Styx has not been as extensive as in the Avon and Heathcote catchments, so the invertebrate fauna here is more diverse, with more of the sensitive EPT animals being found there.  Indeed, a total of 13 EPT taxa have been found in the Styx – 12 caddisfly and the common mayfly Deleatidium.  This latter animal used to be found in the Avon, but has disappeared since the 1990’s, so its importance in the Styx can not be over-emphasised.

Recent assessments of the invertebrate communities in the Styx River to those of other urban and rural rivers around Christchurch showed that the invertebrate community of the Styx is more similar to that of rural rivers near Christchurch, and it is in better health than either urban or semi-urban rivers around Christchurch.  When compared to urban streams nationally, the fauna of the Styx River had some of the highest rankings for the percentage of mayflies and caddisflys.  Many other metrics for describing the invertebrate communities were in the top 20% of values for urban streams nationally.  The only other stream within the greater Christchurch area to support as many EPT taxa was the Otukaikino, which flows through a rural catchment.  Despite these high values, there is strong evidence that sensitive taxa such as the mayfly Deleatidium may be disappearing from the Styx catchment.  The Styx River can be regarded as representing an area of extremely valuable biodiversity within the Christchurch city area, as it supports as many EPT taxa as rural streams.  As such, from a local perspective, the river has high ecological values.  However, the ecological health of this river may be regarded as being in a transitional state between a healthy rural stream, and a less healthy urban stream.