Effect of seagrass patches on macrofaunal communities

Twelve seagrass patches were selected from three sites (Wiroa Island and Clark's beach in Manukau Harbour and the eastern arm of Whangapoua Harbour). These patches were studied at three different spatial scales – rhizome growth patterns and demographics (Manukau sites only), patch dynamics and landscape patterns, to provide an integrated picture of seagrass patch dynamics. Purpose: To determine the influence of wind-wave dynamics and hierarchical arrangements of spatial structure of seagrass plants on macrofaunal communities.

seagrass % cover, seagrass biomass, fractal geometry, patch isolation, mean wind velocity and exceedance.

2 sites in the Manukau Harbour and one site in the eastern arm of the Whangapoua Harbour.

Whangapoua – E2745181 N6493816, NZMG, 1: 20000, AREA

April 1994 – October 1995

Rhizome growth patterns and demographics (Manukau sites only) – Individual rhizomes at 4 positions around the edge of 12 patches at each site were tagged with 5 cm long U-shaped metal pegs placed over each rhizome, proximal to the apical meristem. Lateral growth relative to each tag was measured after 6 months. The growth of each rhizome, as well as the position and length of new branches, and the number of shoot groups distal to the tag were recorded. Additional rhizomes, in different patches at both of the sites, were tagged and measured after one month. Three 0.25 m2 x 10 cm deep quadrats were removed from 5 seagrass patches at each site and measurements taken for length of rhizome internodes; the number, position and length of branches; the number of shoot groups; the number of leaves in each shoot group; and the length of the longest intact leaf. Leaf growth was measured over a 14-day period. 9-12 shoots in each of three patches were marked with a needle through the leaf sheath. Half were retrieved after 2 weeks, and the remainder after a further 10 days. Patch dynamics – Twelve patches (5-7 m mean diameter) were randomly selected from within a 200 m x 200 m plot at each site. Seagrass patch shape, growth and decline, as well as changes in seagrass percentage cover and biomass were measured at 6 monthly intervals in the patches. Shoots and rhizomes were removed from sediment cores (13 cm diameter x 15 cm deep), dried at 60 oC and total biomass calculated. Sorted macrofauna (> 0.5 mm) from these cores was collected and identified. Seagrass cover was monitored photographically using three 1 m2 quadrats and percent cover recorded using 100 random points. Aerial photographs (vertical colour 1:1,500 and colour infrared 1:3,000) were taken in October 1994 and April 1995. Patch area, patch perimeter, and nearest neighbour distances between patches were used as measures of landscape pattern within relatively large areas (2-3 km2) at each of the study sites. Wave exposure values based on mean wind velocity were also calculated for each site, on each sampling occasion. Frequency of collection: 6 monthly

Not reported Gaps in collection: None Data quality: Not reported Attribute accuracy: Not reported Completeness: Not reported

reports, spreadsheets, diagrams

See NIWA Sensitivity/Confidentiality: See NIWA

Finished

Related information: Fish usage of Seagrass study - FRST programme COIX0025 NIWA sediment conditions and seagrass health (FRST 2002 project)