Christine E. Hanson

Seasonal production regimes off south-western Australia : influence of the Capes and Leeuwin Currents on phytoplankton dynamics

Temporal primary production dynamics were investigated off south-western Australia, where the summer upwelling regime of the Capes Current was compared with early winter conditions characterised by strengthened near-shore Leeuwin Current flow. Seasonal upwelling in this region sourced nitrate levels of ≥1 μm from the nutricline at the base of the Leeuwin Current’s mixed layer, with total water column production reaching a maximum of ~950 mg C m−2 day−1 in the Capes Current. Stable isotope signatures of particulate matter indicated that productivity off south-western Australia was heavily reliant on nitrate as a nitrogen source, with mean δ15N ranging from ~4 to 5 ‰ under both upwelling and non-upwelling (winter) conditions. Unexpectedly, significant nutrient enrichment within the Leeuwin Current (up to 3.1 μm nitrate) occurred during winter, likely as a result of the meandering Leeuwin Current flooding the inner shelf north of the study area and entraining relatively high-nutrient shelf waters in its southwards flow. However, early winter production under these nutrient-replete conditions (mean ± s.d. 310 ± 105 mg C m−2 day−1) was significantly lower than in summer (695 ± 140 mg C m−2 day−1) due to light limitation, both as a result of reduced surface irradiance characteristic of the winter months and significantly higher light attenuation within the water column as compared with summer conditions.

AN ECOLOGICAL BASIS FOR EXTRACELLULAR CARBONIC ANHYDRASE IN MARINE UNICELLULAR ALGAE

Extracellular carbonic anhydrase (CAe) is expressed by many, but not all, autotrophic species of aquatic unicellular protists. We measured CAe activities in unicellular marine algae characteristic of either high nutrient spring, fall, and winter blooms or low nutrient summer populations to provide ecological/evolutionary information about the enzyme. Highest activities occurred in spring bloom and opportunistic diatoms exposed to long photoperiods (16 h) when pH was highest and CO2 was lowest. Lower activities were recorded for a fall‐bloom diatom exposed to the long photoperiod, and lowest values were found under all culture conditions for one diatom and a number of flagellated species typical of summer low nutrient environments. Other potential sources of variance in measurements of CAe activity were examined. Maximum activities of CAe were recorded for the diatom, Skeletonema costatum (Greville) Cleve, during late exponential phase of growth and within 8 h of the beginning of the photoperiod. We concluded that ecological factors are important in determining CAe activities in marine unicellular protists. Potential functions of CAe in the metabolism of marine unicellular algae are discussed.

Sustained oceanographic observations around Australia using autonomous Ocean Gliders

Ocean gliders are autonomous sensor platforms designed to operate in water depths of up to 1000m. By varying their buoyancy, gliders are able to efficiently ascend and descend through the water column. The Australian National Facility for Ocean Gliders (ANFOG) has been established as part of the Australian Integrated Marine Observing system to develop and operate a fleet of ocean gliders in shelf and shelf slope waters around Australia. In this paper several glider operations around Australia will be discussed including deployments at the mouth of the Spencer Gulf in South Australia, the continental shelf and shelf slope off Western Australia and the shelf, and East Australian Current eddies off New South Wales. Discussion will focus on WA deployments which represent the first sustained measurements taken in the Southern Hemisphere using ocean gliders.