Circulation and hydrology of Manukau Harbour

116

Exammation of the physiological ecology of 3 sympatncally occurring mytilids (the ribbed, blue and greenshell mussels, Aulacomya maoriana, ivlytilus galloprovincialis and Perna canalicdus, respectively) was carned out during summer and winter at 2 sites which differ markedly in their environmental characteristics (Mahanga Bay inside Wellington Harbour, where mussels are abundant, and Island Bay on the exposed Cook Strait shore, where mussels are absent). Seston components (particle counts per rnl in the size range 2.5 to 63 pm, total particulate matter [TPbI], particulate organic matter [POM], and percent organic matter [PCOM]) all varied significantly between sites and between seasons. Mahanga Bay seston exhibited characteristics typical of productive temperate coastal waters. whereas Island Bay seston was characterised by low POM and low PCOM values. Weight-standardised clearance rate (CRS), net absorption efficiency (AE) and Scope for Growth (SFG) values were determined for all 3 mussel species at both sites during both seasons. At Island Bay there was no evidence of seasonal differences in CRs whereas at Mahanga Bay species-specific CRs values were all highest in the winter. Regardless of season, CRs estimates for all 3 species were higher at Mahanga Bay than at Island Bay. At both sites, AE estimates were highest in the w~nter, and AE estimates were higher at Mahanga Bay than at Island Bay during each season SFG estlrnates mirrored AE estimates, with SFG at both sites being highest in winter, and higher at Mahanga Ray than at Island Bay dunng each season. The SFG data support the hypothesis that mussels are absent from sites along Cook Strait shores because they do not enjoy a positive energy balance for long periods of time, consistent wlth the low seston organic matter levels at such locations. The SFG data also indicate that mussels lnside Wellington Harbour can maintain a large, positive net energy balance throughout the year, consistent with the considerable biomass and very large size of all 3 species in this environment.

Section: Discussionmentioning

confidence: 99%

Where are the mussels on Cook Strait (New Zealand) shores? Low seston quality as a possible factor limiting multi-species distributions

Gardner¹

2000

116

“…Boccardia syrtis), while predictable with flushing is relatively high as a result of the fact that a regard to the annual timing of their occurrence, were large proportion of the Harbour is emptied at every unpredictable in terms of their magnitude. Large low tide (Heath 1975, Heath et al 1977 The prevailing wind direction over Manukau Hardia syrtis interactions inhibit the recruitment of PI. lilbour is from the southwest and west in all seasons, iana and have contributed to the low abundances of winds from the northeast are not uncommon (Fig.…”

Section: Evaluation Of Community Persistence and Variabilitymentioning

confidence: 99%

Are soft-sediment communities stable? An example from a windy harbour

Turner¹,

Thrush²,

Pridmore³

et al. 1995

The macrobenthic soft-sediment communities at 6 sites in Manukau Harbour, New Zealand, have been quant~tahvely sampled at bimonthly intervals since October 1987. Univariate (contingency-table analysis, Kendall's coefficient of concordance and Jaccard's similarity index) and multivariate (non-metric multi-dimensional scaling and canonical con~munity ordination) analyses were used to assess the absolute and relative structural stability of the communities a t each of the 6 sites over the first 5 % yr of sampling. Despite short-term, seasonal and inter-annual variability, the communities have generally been persistent, in terms of their overall community structure, over the duration of the sampling programme. h4anukau Harbour is a dynamic and rigorous environment in terms of movement of water and sediments. Mean wind c o n d~t~o n s were found to play an important role in contnbuting to variability in comn~unity structure at different sites in the Harbour, probably as a result of s e h m e n t disturbance by wind-induced waves and bottom turbulence. In addition to local factors, there was also evidence that factors operating at harbour-wide scales (e.g. water temperature) contributed to variability in community structure. The populations of some species exhibited marked temporal fluctuations. Major recruitment events potentially lead to altered physical, chemical or biological characteristics of the benthic habitat with concomitant effects on other species in the community. Despite the physically and biologically dynamic environment, the communities were persistent and exhibited both resistance? and resilience to physical disturbance and major recruitment events. Other studies have reported these to b e important factors contributing to instability in the structure of soft-sediment communities.

“…85 km 2 ), linked in the south to coastal Cook Strait via a single channel. Mean water depth of the harbour is 14 m, with a maximum depth of 32 m south-west and south-east of Matiu-Somes Island (Heath 1977). Tides are semi-diurnal and small with a maximum amplitude of 1.5 m.…”

Section: Methodsmentioning

confidence: 97%

Recovery of a subtidal soft-sediment macroinvertebrate assemblage following experimentally induced effects of a harmful algal bloom

Kröger

Gardner²,

Rowden³

et al. 2006

A defaunation experiment mimicking the effects of a harmful algal bloom (HAB) on benthic macroinvertebrate assemblages (> 500 µm) was conducted at a hydrodynamically active soft-substrate site in Wellington Harbour, New Zealand, to test the recovery rate (return to predisturbance state) of temperate benthic macroinvertebrate assemblages and to elucidate the main factors influencing the recovery process. Tarpaulins were used to create anoxic conditions by smothering the sediment for 65 d. Assemblage recovery in treatment plots was studied for 1 yr and compared with assemblage composition in undisturbed adjacent control plots. Recovery was slow until Day 70, at which time abundance of individuals (N) and number of species (S) increased synchronously in treatments and controls. Within 10 mo, univariate indices (N, S and also species diversity H' and evenness J') of treatment assemblages returned to values observed for the control assemblages. Multivariate analyses showed that fluctuations in assemblage composition were most pronounced in the first 100 d in treatment replicates, but decreased thereafter as the recovering assemblage became more similar to the undisturbed one. However, after 1 yr, even though the composition of treatment and control assemblages was converging, differences in composition were still significant. Based on the observed trajectory of recovery, complete assemblage recovery is predicted to take approximately 2 yr. Timing of the disturbance in relation to seasonal recruitment events was identified as an important factor for assemblage recovery.