Alex H. Ross

A dynamic energy budget model of the Pacific oyster Crassostrea gigas

A dynamic energy budget model has been developed to simulate the growth of Pacific oyster Crassostrea gigas in response to varying environmental conditions. The model is designed to incorporate the effects of endogenous (core weight and storage) and exogenous (temperature, quantity and quality of food) factors and to be applicable to a variety of ecosystems. Two state variables (core weight and storage) are used in order to avoid confusion between functional roles of structural and reserve tissues. Assimilation and metabolic rates are modelled as functions of core weight, while reproduction is entirely dependent of storage. In addition to reproduction, a variable in terms of energy requirements for gametogenesis is introduced in this model. Calibration of the model was done through sensitivity analysis and comparison of simulation outputs and observed data. The model is capable of simulating growth and condition of oysters in the ecosystem of Marennes–Oleron Bay. The simulations indicate that growth of oysters is strongly regulated by the phytoplankton concentration, while detritus has little contribution.

Phytoplankton biomass associated with mussel farms in Beatrix Bay, New Zealand

The spatial and temporal variability in phytoplankton abundance is a major factor determining the productivity of mussel (Perna canaliculus) farms. During periods of low phytoplankton abundance, food depletion may be a significant factor in the productivity of mussel farms. Measurements of phytoplankton abundance (as chlorophyll a) were made over the entire depth of the water column both inside and outside four mussel farm sites in Beatrix Bay, over a 13-month period. Ambient (outside) concentrations of dissolved inorganic nitrogen (NO3–N and NH4–N) and dissolved reactive phosphorus (DRP) were also measured. The highest ambient chlorophyll a concentrations in the surface waters occurred during autumn–winter, with peak levels of around 5 μg l−1. At this time, all farms had a significant reducing impact on phytoplankton biomass (P<0.05 in all cases). The lowest ambient chlorophyll a concentrations of <0.5 μg l−1 were recorded in the summer, coinciding with low nitrogen concentrations. In November, there was significantly more phytoplankton inside all the farms (P<0.05 in all cases). It was hypothesised that this increase occurred because mussels are net producers of dissolved inorganic nitrogen. In five out of the 7 months when farms were surveyed, the highest chlorophyll concentrations were in the deeper water, associated with a pycnocline. Two management options to increase mussel productivity are presented: (1) Deployment of mussel dropper ropes to deeper waters to take advantage of chlorophyll maxima in summer; (2) Artificially increasing nitrogen inside farms during spring and summer to increase phytoplankton supply.

Seasonal variation in the reproductive activity and biochemical composition of the Pacific oyster (Crassostrea gigas) from the Marlborough Sounds, New Zealand

Abstract Reproductive cycles of the Pacific oyster Crassostrea gigas (Thunberg) from the Marlborough Sounds, New Zealand, were followed between June 1998 and January 2000. Histological examination of the gonad confirms an annual cycle with a winter inactive period followed by rapid gonad development and a single short spawning period. The population gonad index correlated with seawater temperature and changes in tissue dry weight, condition index, and biochemical components. In winter, few individuals with early gametogenic stages were present and rapid development of primary oocytes (diam. 11 μm) occurred during spring (September‐November). The developmental rate and the diameter of mature oocytes (37 μm) was similar for the 1998 and 1999 seasons. For a standard 110‐mm‐length oyster, maximal tissue body weight and condition index were recorded in December. Rapid weight loss in January was length dependent and was attributed to spawning. Temperature was the environmental variable which best correlated with the timing of gametogenesis. Food availability (phytoplankton biomass) may have been responsible for inter‐annual variations. The biochemical composition (% glycogen, lipid, protein) of separated gonad and somatic tissues were variable seasonally and annually. Gametogenesis (oocyte diameter) was associated with increased gonad protein and glycogen and a decrease in lipid concentrations. These changes are similar to those in Pacific oyster populations from other parts of the world.

Size‐specific clearance, excretion, and respiration rates, and phytoplankton selectivity for the mussel Perna canaliculus at low levels of natural food

Abstract Clearance, respiration, and ammonia excretion rates, and phytoplankton selectivity were measured for the Greenshell™ mussel Perna canal‐iculus (Gmelin) in Beatrix Bay, Marlborough Sounds, New Zealand. Functional equations were developed describing the relationship between body size (length and weight) and clearance and metabolic rates for this species feeding on natural food assemblages in New Zealand waters. Relationships with shell length were described by allometric power curves but the relationship with chlorophyll a concentrations in the water for a particular size of mussel suggested there was a threshold between 0.2 and 0.3 μg Chi. a litre−1 and an exponential decline in clearance rate from c. 0.5–0.6 μg Chi. a litre−1 with increasing chlorophyll concentrations. Under these field conditions ingestion rate would increase linearly with increasing chlorophyll concentrations up to c. 0.5 μg Chi. a litre−1 then remain constant at higher chlorophyll levels. Food levels, as measured by chlorophyll a or particulate carbon, were very low (generally <1 μg Chi. a litre−1 and 300 μg C litre−1) throughout the study and this was reflected in low ingestion rates, despite clearance rates of up to 8.6 litre mussel 1 h−1. P. canaliculus generally appears to be non‐selective, at least for phytoplankton in the size range 5–100 μm, but there was some evidence that larger diatoms and dinoflagellates were cleared more efficiently by larger mussels. Some of our observations on food levels and feeding rates help explain the low mussel growth and condition experienced in the Pelorus Sound during 1996–97.

COMPARISON OF ASSIMILATION EFFICIENCY ON DIETS OF NINE PHYTOPLANKTON SPECIES OF THE GREENSHELL MUSSEL PERNA CANALICULUS

Abstract The greenshell mussel Perna canaliculus is the most important species in aquaculture in New Zealand. Mussel energetics and growth rates are subject to the natural variability in phytoplankton biomass and species composition and thus understanding the influence of food type on assimilation efficiency is fundamental to the prediction of mussel production and planning farm management. In this study pulse-chase feeding techniques were used to assess the effect of diet on assimilation efficiencies for nine phytoplankton species including three diatoms Chaetoceros calcitrans, Skeletonema costatum and Thalassiosira sp., and three flagellates Eutreptiella sp., Pyramimonas sp. and Isochrysis galbana, and three dinoflagellates Akashiwo sanguineum, Alexandrium minutum and Gymnodinium catenatum. Assimilation efficiency varied with algal species, but it was significantly higher when mussels were fed dinoflagellates (84.5%) compared with diatoms (61.7%) and flagellates (77.9%). Assimilation efficiency of dinoflagellates and flagellates increased with gut passage time, whereas with a diatom diet, a negative correlation was evident. This finding has implications in understanding and predicting growth rates of mussels (and hence commercial yield) in tandem with natural variability in phytoplankton species composition.

Abrupt population changes along smooth environmental gradients

Populations often exhibit abrupt changes in abundance associated with a smooth, continuous change in some component of their environment, with the abruptness usually attributed to inter-specific interactions or physical extremes. This paper presents a spatially explicit single-species population model in which intra-specific interactions alone are responsible for such an abrupt change. The essential mechanism involves cooperation in both colonization (through enhanced recruitment near other individuals) and mortality (protection through a “safety-in-numbers” interaction). Large fluctuations in population density would likely be observable near the transition region.

In situ enclosure experiments on the influence of cultured mussels (Perna canaliculus) on phytoplankton at times of high and low ambient nitrogen

Abstract The influence of the cultured mussel Perna canaliculus (Gmelin 1791) on the abundance of phytoplankton was investigated in Pelorus Sound, New Zealand. Four in situ enclosure experiments were undertaken, two in summer when ambient nitrogen was low, and two in winter when it was high. Each experiment had four manipulation types: added mussels; added nitrogen; both mussels and nitrogen added; and control (no additions). In summer, there was a significant increase of chlorophyll a in response to added nitrogen, indicating that the phytoplankton were nitrogen-limited. At this time, mussels caused an increase (11–17%) in phytoplankton biomass, possibly by converting particulate nitrogen to ammonium, making the nitrogen available for phytoplankton utilisation. The highest ambient chlorophyll a concentrations coincided with high ambient nitrogen in the winter. At this time, mussel grazing caused a significant decrease (5–14%) in phytoplankton concentration, indicating that within-farm depletion of phytoplankton is most likely to occur in winter. On an annual time scale, the mussels had a stabilising influence on phytoplankton biomass, reducing high ambient levels in winter and slightly increasing low levels in summer.

Nutrient cycling and fluxes in Beatrix Bay, Pelorus Sound, New Zealand

Abstract Nitrogen (N) and light availability limit phytoplankton growth and hence regulate food supply for mussel farming in Pelorus Sound, New Zealand. In Beatrix Bay, a large 35‐m deep embayment off Pelorus Sound, the supply of N is heavily influenced by physical factors. Bottom intrusions of N‐rich or depleted oceanic water occur in the main channel of Pelorus Sound and N‐depleted, phytoplankton‐enriched Pelorus River water characterises the near‐surface low salinity field. Inside Beatrix Bay, nutrient cycling via phytoplankton uptake, sedimentation, and subsequent regeneration from the sediments by mineralisation, also has an important influence on the availability of N in the water column. However, almost continuous salinity or thermally induced density stratification, or both, within Beatrix Bay meant that the upper and lower water columns were often decoupled so that N regenerated from the sediments was not readily available in the upper water column, and phytoplankton settling from the upper water column may not reach the sediments directly below because of large horizontal transport. The N supply in the upper water column of Beatrix Bay, where the mussels are farmed, is most likely regulated by advection in and out of the bay from external sources. Conversely, the N concentrations below the pycnocline could be almost entirely supplied via mineralisation from the sediments. The rates of sediment N regeneration were seasonally dependent. During winter, when sedimentation exceeds mineralisation, N accumulates in the sediments at a rate of up to 10 mg m–2 day–1, whereas in summer, when mineralisation exceeds sedimenation, N is lost from the sediments at rates of up to 9 mg m–2 day–1.

Evidence of a significant wind‐driven circulation in Akaroa Harbour. Part 1: Data obtained during the September‐November, 1998 field survey

Abstract Part 1 (P1) of a two‐part study of Akaroa Harbour, Banks Peninsula, New Zealand, presents observational evidence of the development of a significant wind‐driven circulation in association with strong northerly and southerly wind events. These are common throughout the year though during the 3‐month field campaign (during the Southern Hemisphere spring of 1998) northerly wind events dominated. This wind‐driven circulation resulted in the weakening of density stratification within the Harbour, mixing the water column to depth over a significant portion of the study area.

Evidence of a significant wind-driven circulation in Akaroa Harbour. Part 2: Results from the application of a width averaged hydrodynamic numerical model (CE-QUAL-W2)

Abstract Results from the application of the two‐dimensional, laterally‐averaged hydrodynamic model, CE‐QUAL‐W2 to Akaroa Harbour, New Zealand were compared with 3 months of temperature and current data to investigate the connection between significant wind‐driven circulation and vertical mixing. Results indicate that in general, CE‐QUAL‐W2 reproduced realistic values for width‐averaged along‐Harbour current speeds and water column temperature structure. However, during periods of strong winds, the mixed layer depth was predicted to be shallower than observed; c. one‐third as compared to c. one‐half. Simulations of tidally‐driven flow highlighted the restrictions of the two‐dimensionality imposed by the model, though this type of flow was not found to result in appreciable mixing of the water column. Data from an independent field survey supported the validity of the use of CE‐QUAL‐W2 under (at least) light‐to‐moderate sea‐breeze conditions.