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«A summary document prepared on behalf of the Mangrove Steering Group. Current and Recent Research Activities Auckland University of Technology Dr. A. ...»

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July 2006

A summary document prepared on behalf of the Mangrove Steering Group.

Current and Recent Research Activities

Auckland University of Technology

Dr. A. C. Alfaro, Earth and Oceanic Sciences Research Centre

Below is the


of a paper which summarises the research by Andrea Alfaro.:

Alfaro, A.C. (2006) Benthic macro-invertebrate community composition within a mangrove/seagrass estuary in northern New Zealand In the tropics and sub-tropics, estuarine environments with mangrove and seagrass habitats provide important structures and resources for diverse communities of benthic organisms.

However, temperate estuarine habitats, especially in mangrove areas, may differ significantly in their community associations and interactions. The community composition of benthic macro-fauna was investigated within temperate Matapouri Estuary, northern New Zealand.

The density and distribution of fauna were sampled within six distinctive habitats (mangrove stands, pneumatophore zones, Zostera beds, channels, banks and sand flats), within four sampling events between December 2002 and September 2003. Each type of habitat was replicated seven times within different locations in the estuary. Counts of all infauna and epifauna within four replicate cores were recorded from each habitat and location.

Multidimensional scaling plots were used to identify differences in structure and composition of assemblages among habitats and locations within each sampling event. Results from these benthic samples indicate that Matapouri Estuary has a high overall biodiversity, with distinctive faunal assemblages found within different habitats, and some seasonal variations also apparent. In terms of both number of individuals and taxa per unit area, seagrass beds had the highest numbers and mangrove areas had the lowest numbers, with all other habitats in between. Some locations were found to support a high diversity of organisms across habitats, while other locations had high densities of a few species only. Several physical and biological differences between tropical/sub-tropical and New Zealand's temperate mangrove habitats are put forth as potential reasons for the lower density and diversity of the benthic component observed herein. Further on-going studies aim to elucidate the structure and interactions within food webs in this estuarine ecosystem.

Further details about the research programme undertaken by the AUT are presented below.

Introduction Complex food webs and ecosystems can be found in estuarine environments. The diversity of habitats and organisms in estuaries is a result of the dynamic interaction of physical and biological components between marine and terrestrial realms. The influence of natural and anthropogenic activities, such as catchment runoffs, tidal regimes, animal movement,and organic matter accumulation, are likely to directly or indirectly affect each and every habitat along the estuary. Furthermore, inputs from catchments and the open ocean may vary from one estuary to another and throughout time.

At Matapouri, two major catchments feed into the estuary. The southern catchment feeds into the Parangarau stream and the northern catchment into the Te Wairoa stream (Figure 1). The two streams meet near the mouth of the estuary, which opens to Matapouri Bay (Figure 1). The tidal influence is strong, and has an effect into the upper reaches of the estuary. The freshwater inputs are relatively low, unless heavy rain has occurred. The interconnectedness of estuarine dynamics, from catchment to the sea, has provided a platform for ongoing research at Matapouri since early 2002 by the Marine Ecology Group at the Auckland University of Technology. This study site was chosen for its small size (manageable field activities), and its high habitat and species biodiversity (the two streams and their associated habitats provide great replication potential).


The overall aims of this research programme are numerous, and are documented as follows:

1. Characterise the physical components (i.e. water flow regime, sediment types, nutrient composition, oxygen concentration, pH, temperature) and biological components (i.e., benthic macro-invertebrates, fish populations, plankton concentrations, insect and bird populations) within the various habitats in the catchment, estuary, and coastal areas. \

2. Identify any sources and/or sinks for nutrients, energy, and sediments among the catchment, estuary, and coastal environments.

3. Elucidate and model the trophic level, and food web dynamics in the estuarine ecosystem.

4. Investigate life history strategies for key estuarine species (i.e. crabs, snails, barnacles, oysters, pipi, cockles).

5. Determine the sedimentary fill history and geologic-geomorphic development of the estuary.

6. Identify any potential pollution (i.e., heavy metals, coliforms) sources and sinks within the Matapouri system.

7. Develop a GIS database of all physical and biological components of the Matapouri system.

8. Develop a series of sustainable community-based environmental management strategies for the Matapouri area, in conjunction with local Maori and other communities.

9. Develop and implement a knowledge-based framework of the Matapouri coastal area that incorporates mätauranga pütaiao and western science.

Summary of research The following research projects have been conducted/or are currently being conducted/or are planned by various researchers and students from the Auckland University of Technology, University of Auckland, and Northland Polytechnic. These projects have been designed to feed into the overall research programme aims which will provide a comprehensive and multidisciplinary approach to our investigation of estuarine system dynamics in northern New Zealand. The majority of the research is unpublished, with many initial publications in preparation, although several students have submitted conference abstracts, reports, and thesis’s (currently held at the Auckland University of Technology and University of Auckland libraries). Our research team currently undertakes research projects in other estuaries in northern New Zealand. However, only the projects based within the Matapouri Estuarine System are included in this summary. A certain degree of emphasis has been placed on the distribution of benthic fauna within the estuary. Further information on other aspects of this programme is available upon request.


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Figure 1: Map of Matapouri showing the two streams (Parangarau and Te Wairoa), and the locations of 7 sampling transects. Modified from Morgan (2003).

Benthic macrofauna characterisation at Matapouri The density and distribution of macro-invertebrates within freshwater streams in the two subcatchments were sampled over a period of approximately 8 months throughout 2003 and 2004 (Pohe et al., 2002). Ten stream sites were sampled within the native bush, and pasture areas (Figure 2). A Macro-invertebrate Community Index (MCI) was generated and used to evaluate the degradation category of each sampled site. The results indicated that the MCI and total number of taxa are generally higher in the Te Wairoa sub-catchment than the Parangarau sub-catchment. The results also demonstrated that the native bush habitats had significantly higher total numbers of taxes and lower degradation indices than pastoral land.

These biological trends correlated well with the physiochemical parameters measured at each site.

Benthic macro-invertebrates were sampled at seven locations within the two arms of the Matapouri Estuary (Figure 1). Seven distinctive habitats were identified and sampled with seven transect lines. These habitats were; mangrove stands, pneumatophore zones, seagrass beds, subtidal channels, banks, sand flats, and dunes. Four comprehensive sampling events were undertaken in December 2002, and March, June, and September of 2003 within all habitats and transect locations (Alfaro, in preparation; McDowell and Alfaro, 2003; McDowell, 2003). In addition, sub-sampling events have been conducted since early 2002.

Results from these investigations indicate that each habitat contains a unique benthic community assemblage, and that the total number of benthic invertebrates and number of taxa vary among locations (Alfaro, in preparation). Mangrove habitats tend to have a low species diversity and abundance, although some cockles and oligochaete worms are found in the sediment. Oysters and barnacles can be found around tree trunks and the scattered pneumatophores (present in low numbers within the mangrove stands). The pneumatophore zone (between the mangrove stands and the channel) has a high diversity of oysters, mussels, and barnacles. The sediment in this area often contains an average number of cockles and snails (scavengers and herbivores). The seagrass bed habitats are by far the most diverse habitat in the estuary, containing the largest numbers of species. High numbers of small (juvenile) cockles are present in these habitats. It is also likely that cockles are recruiting in these seagrass areas.

Figure 2: Location of sampling sites within the Matapouri sub-catchments. From Pohe (2003).

The recruitment sites of cockles are of particular interest because they are relatively poorly understood. Juvenile pipi, wedge shells and other bivalves, crabs, shrimp, and a range of carnivorous and herbivorous snails often are found in high numbers in these habitats. The channel habitats are dominated by pipi, cockles, and oligochaete worms. At high tide, a large number of fish come into the estuary. Juvenile and adult fish have been observed feeding in the seagrass and mangrove areas, even far into the upper estuary. The bank habitats tend to have a high density of large and small pipi. Snails, such as Cominella glandiformis, Diloma subrostrata, and Melagraphia aethiops, are often found in these habitats in high numbers.

The sand flat areas have patchy distributions of pipi, cockles, worms, amphipods, and snails.

At the water table within these habitats, juvenile pipi can often be found, especially after spawning periods. The dune habitats generally have a low diversity of benthic invertebrates, but tend to support larger numbers of isopods, beetles, and other terrestrial insects. Thus, relative comparisons among habitats indicate that seagrass habitats have the highest biodiversity and abundance of benthic organisms, while dunes and mangrove habitats have the lowest diversity and abundance of benthic fauna.

Comparisons among locations indicate that site 2 (near the northern bridge, along the Te Wairoa stream) had the most extensive seagrass bed in the estuary, and that this area also has the highest diversity and abundance of benthic invertebrates (both within the seagrass beds and also within adjacent habitats (Alfaro, in preparation)). Site 6, along a small branch of the Parangarau stream, had the second highest biodiversity and abundance of organisms among all locations. A relatively high density of cockles also can be found in the sand flat habitat at site 6. While the sand flat, banks, and channel habitats generally had lower biodiversity than the seagrass and pneumatophore areas, these habitats can at times, contain high densities of bivalves and snails. Specifically, site 4 (near the mouth of the estuary) often yielded high abundances of large pipi. The dynamics of this pipi dispersal and recruitment is currently under investigation by Byron Maxwell (AUT postgraduate student).

Although the results from this work are not yet available, observations over the past two years indicate that juvenile pipi are highly mobile within sites 3 and 4. By using their byssal threads, pipi can move easily in between the water table areas (high sand flats) and the bank/channel areas. This movement tends to have a seasonal pattern, which is generally relative to spawning periods, but may happen within a tidal cycle. The movement of pipi, at Matapouri, which sometimes includes a large portion of the population, is likely to be related to settlement cues, lunar cycles, and tidal regimes. Further investigations on food availability and condition index will be undertaken by Byron Maxwell in the coming year.

Phytoplankton and zooplankton dynamics are currently being investigated by Peter Conway (AUT postgraduate student). The preliminary results indicate that there is great seasonal variation in plankton availability at Matapouri, which may directly affect the food resources and settlement opportunities of the estuarine fauna.

Preliminary insect surveys within mangrove habitats indicate that a greater insect biodiversity may be found in the Te Wairoa arm along mature mangrove trees, compared to younger trees in the Parangarau arms of the estuary (Hudson and Alfaro, unpublished data). Bird surveys are being undertaken by Oliver Ball (Northland Polytechnic).

Nutrient and sediment dynamics

Macro-nutrient and micro-nutrient concentrations have been measured throughout the catchment and estuary over time (Soliman and Alfaro, 2003). Great variability in nutrient concentrations have been found among sub-catchments and arms of estuarine channels.

Most of this variability has been attributed to catchment input, although some oceanic inputs have been recorded by Nabil Soliman (AUT postgraduate student).

The nutrient inputs of shore-cast algae (mostly red and brown algae from the open ocean) and resident Hormosira banksii have been investigated (Moy, 2003; Pope, 2003). We have determined that shore-cast algae provide nutrient inputs to the estuary either seasonally or after storms (Alfaro, unpublished data; Pope, 2003). Furthermore, H. banksii is also a permanent and dense source of nutrients for many herbivores, such as Turbo smaragdus and Melagraphia aethiops (Alfaro, unpublished data; Moy, 2003).

The rate of nutrient contributions from Mangrove plants to the estuary has been identified to vary among locations in the estuary (Harvey, 2003). The pathways and rates of organic matter incorporation, via mangrove and seagrass into various estuarine food webs is currently being investigated (Alfaro, unpublished data).

Food web dynamics

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