Report: TR 2008/31

Author: Mike Scarsbrook, Aslan Wright-Stow, Kristel van Houte-Howes and Kurt Joy (National Institute of Water and Atmospheric Research (NIWA))

Abstract

The karst landscape of south-western Waikato (Maniapoto Karst), created through limestone dissolution, is one of the region’s iconic landforms. The caves, springs and other features associated with this landform are valued for their association with Ngati Maniapoto culture, unique natural heritage attributes, utility for recreation and tourism, and their role in supply of water. Despite their regional significance, our knowledge of the structure and functioning of karst aquatic ecosystems is limited and this may constrain effective management of aquatic ecosystems in karst landscapes.

In this report, we summarise results of extensive surveys of aquatic invertebrates and water quality in cave and spring habitats of the Maniapoto karst area. The aims of this study were to relate biodiversity patterns and water quality to a range of potential environmental drivers, including water source (i.e., groundwater vs. surface water), land use and tourism activities. In addition, a comparison was made between springs draining the Maniapoto Karst with those found in a diverse range of lithologies to assess the distinctiveness of karst habitats. The final aim of the study was to provide recommendations on approaches to managing aquatic habitats in karst landscapes.

There are several distinct aquatic habitats associated with karst. We sampled spring, side passage and cave streamway habitats, but did not sample the epikarst (saturated zone between the surface and the cave, often containing aquatic fauna). Habitat type (spring, streamway or side passage) was an important driver of observed patterns in both physicochemical and biological data from aquatic ecosystems of the Maniapoto Karst. Temperature and conductivity/alkalinity provided useful indicators of the source of water, with observed patterns suggesting that autogenic (groundwater-dominated) habitats (i.e., side passages and some springs) can be characterised by lower temperatures and higher alkalinity. Differences in water source were also reflected in the fauna present in springs and side passage habitats. In particular, the stygobitic (groundwater) amphipod Paraleptamphopus sp. was an indicator of habitats dominated by groundwater sources. Very little is known of the life history or feeding ecology of this invertebrate. We suggest that future research into its population ecology would provide valuable information to assist the management of karst ecosystems dominated by groundwaters.

In contrast to water source, land use patterns seemed to have limited influence on biological patterns at sampled sites, although pasture-dominated land use was associated with elevated temperature and nitrate concentrations in each habitat type. In addition, our surveys showed no evidence of adverse effects of tourism activity on cave streamways. We suggest that the absence of strong land use effects on biological patterns may be the result of cool groundwater inflows acting as a buffer against elevated temperatures (compared to surface flowing streams) that might otherwise act as a major stressor on invertebrate communities.

Effects of tourism on cave streamways ecosystems are likely to be minor, due to low numbers of invertebrates in these systems and the relatively small proportion of habitat actually directly affected by human traffic. However, further research will assess indirect tourism impacts on native fish which are known to occur in underground cave systems in the area. The potential impacts on fish may be greater than for other ecosystem components, because some cave operators have installed physical barriers that might limit fish passage.

Spring invertebrate community structure in Waitomo was distinct from that in sites from a number of different lithologies around the Waikato (Kaimai ignimbrite, Waihou sedimentary sands/ashes, Kawhia sands, Lowland sedimentary), and waters from these systems tended to exhibit low temperature and high conductivity.

Despite the significance of karst landscapes to the Waikato Region there are no objectives in the Regional Plan that relate specifically to water-related issues in cave and karst environments. It is assumed that achieving general objectives related to water quality, aquatic ecosystems and water quantity will ensure that water in the Region’s karst areas is managed appropriately. Based on results of this study, we suggest that Environment Waikato’s Regional Plan is likely to be providing adequate implicit protection for most karst aquatic ecosystems from catchment land use activities, because objectives relating to erosion control, improved point source treatment and riparian management are all likely to benefit karst aquatic ecosystems. However, under Variation No. 6 (Water Allocation) of the Plan, water flowing in karst systems is considered to be surface water rather than groundwater. We suggest that this definition fails to recognise the presence of important groundwater habitats and the distinct communities associated with them. We recommend that identification of significant areas of autogenic karst and associated anthropogenic threats to natural heritage values should be a priority for future management-driven research within the Maniapoto Karst.

Aquatic Ecosystems of the Maniapoto Karst [PDF, 1.3 MB]