Offshore Seagrass Beds

This study aimed to determine the extent and biomass of the known sub-tidal seagrass beds around Slipper Island, and the historically reported sub-tidal seagrass beds around Great Mercury Island, both of which fall within the boundary of Waikato Conservancy, Department of Conservation (DOC). The aim is to increase understanding of the role that these rare habitat types play in the ecosystem, which in turn will enhance the Department of Conservation's ability to set environmental targets for the restoration of sub-tidal seagrass at locations where it is thought to have existed previously. Permanently submerged beds of seagrass (Zosteraceae) in coastal waters are rare in New Zealand, where most seagrass beds are confined to the intertidal zone of estuaries. This study describes some environmental conditions associated with submerged seagrass beds at Slipper and Great Mercury Islands. Field work was carried out in May–June 2004. The seagrass bed in South Bay, Slipper Island, is permanently submerged, grows to 4–5 m below chart datum, and covers an area of approximately 0.03 km2. In contrast, seagrass in Huruhi Bay, Great Mercury Island, was estimated to cover an area of approximately 0.07 km2, and the bed was more characteristic of mainland estuaries, with an intertidal component and a sub-tidal fringe to 1 m below chart datum. At Slipper Island, long leaves (up to 47 cm) combined with high percentage cover and biomass (74–229 g dry weight/m2) provided a substantial three-dimensional habitat, supporting a higher macroinvertebrate abundance and diversity than Huruhi Bay or any of the Coromandel Peninsula sites previously reported. The two island locations supported fish assemblages that differed substantially from their mainland, intertidal counterparts. Huruhi Bay supported high abundances of exquisite goby (Favonigobius exquistes) and sand goby (F. lentiginosus), juvenile yellow-eyed mullet (Aldrichetta forsteri) and snapper (Pagrus auratus), with juvenile snapper densities being the highest ever recorded over seagrass in New Zealand. In contrast, South Bay supported a substantial population of an undescribed pipefish (Stigmatopora cf. macropterygia), but only low numbers of other fish species. The Slipper Island site is an excellent example of the high potential ecosystem value of sub-tidal seagrass beds.

This study describes characteristics of the seagrass beds and some relevant environmental conditions where the beds are found, and assesses the diversity of associated fish and macroinvertebrates. Results are compared with historical data and with existing knowledge of habitat characteristics of intertidal beds in nearby Coromandel estuaries. Detailed results and discussion in report.

Subtidal areas around Great Mercury Island and the Slipper Island group.

May 2004

Three temporary 50-m-long transects were laid within each seagrass bed (perpendicular to shore). At Slipper Island, % seagrass cover was estimated within a 1-m2 quadrat at 2-m intervals along the transect by a SCUBA diver. Where visibility was high, video was also taken at a fixed height of 70 cm above the bottom. % cover estimated to nearest 5%, and estimates placed into the cover scale of Braun-Blanquet (Braun-Blanquet 1932). The technique involves estimating percentage cover within five cover classes: 1 = 1%–5%; 2 = 6%–25%; 3 = 26%–50%; 4 = 51%–75%; 5 = > 75%. Four cores taken at each transect for further analysis, including grain size, biomass and macroinvertebrates. The small-fish assemblages present at the two locations were sampled using a standardised beach seine, and also surveyed using SCUBA visual surveys. Full details in report. Depth profiles of photosynthetically available radiation (PAR) were made using a PUV500 profiler (Biospherical Instruments Inc.) on 4 May 2004 at Slipper and 5 May 2004 at Great Mercury. To further characterise water clarity in the region, depth-profiles of PAR were measured at six locations between Tairua Harbour and Slipper Island on 24 August 2004. Frequency of collection: One-off

Time constraints precluded an extensive survey of bays other than those where submerged seagrass had previously been reported. The area of the seagrass beds could only be estimated in this study owing to limited time and weather constraints on the work. Maximum depth boundaries were determined by snorkel or SCUBA divers as the point where seagrass cover exceeded 5%. This decision rule underestimates the potential niche available for seagrass growth (i.e. some plants will extend beyond this point). An additional source of error in this estimate is the fact that observing minimum and maximum depths alone does not account for bare patches within the bed.

.pdf report

Report available.