PHOTOPHYSIOLOGY OF A TURF ALGAL COMMUNITY: INTEGRATED RESPONSES TO AMBIENT LIGHT AND STANDING BIOMASS(1).

This study investigated the variation in the relationship between photosynthesis and ambient light (P-E curves) for turf algal communities on a temperate reef off the coast of South Australia, analyzing the integrated effects of ambient light and standing biomass. The photophysiology of turfs was studied in situ on a seasonal basis, examining algal communities growing on artificial substrate (plates) at depths of 4 m and 10 m. P-E curves and estimates for the photokinetic parameters (Pm , Rd , α, Ek , and Ec ) were obtained through oxygen evolution methods, using an automated underwater respirometer. Photoacclimation responses to changes in ambient light were strongly affected by the biomass of the community. Pm showed an inverse relationship to standing biomass, irrespective of depth and season, which was considered to be a response to self-shading and boundary layer effects. Biomass effects imposed a high variance on estimates for all photosynthetic parameters, overshadowing differences observed for season and depth. Biomass also affected photoinhibition on turf communities, where significant afternoon depression of photosynthesis was observed in sparse turf patches when compared to denser patches. High areal productivity rates were maintained across all seasons with a significant decrease only being observed during winter.

Preliminary model of nitrogen loads from southern bluefin tuna aquaculture.

Farming of wild tuna in coastal areas is a relatively new aquaculture industry and little is known about the magnitude of nutrient discharges to the environment. In this work we present a preliminary model of nitrogen loads from southern bluefin tuna (Thunnus maccoyii) aquaculture in lower Spencer Gulf, South Australia. The model was developed based on feed inputs, estimates of fish metabolism and environmental data. Two pens were monitored over a full grow-out season to determine nitrogen sedimentation fluxes, remineralization at the sediment-water interface and accumulation in the sediments. The model suggests that the high metabolic rates of tuna lead to low retention of nitrogen in fish tissues (7-12% of feed inputs) and high environmental losses (260-502kg Ntonne(-1) growth). Considering Australian annual production of 4380tonnes over initial stocked biomass, total loads can reach 1137tonnes N per year, 86-92% lost as dissolved wastes. The nature of wastes suggests low localized impacts at current stocking densities and holding periods.

Test of an antifouling treatment on tuna fish-cages in Boston Bay, Port Lincoln, South Australia.

A test of the antifouling properties of Wattyl NetClear was conducted on fish farms containing southern bluefin tuna (Thunnus maccoyii) during a 6-month period (February-July 2002). Wattyl NetClear is a water-based synthetic latex-based coating where the active antifouling agent is a mixture of two isothiazolinones likely to affect both biochemical conditioning and bacterial colonisation. The development of fouling was monitored using underwater stereo-photogrammetry on 3 treated and 3 untreated net cages with the main factors treatment, depth and cage nested within treatment. A significant treatment and depth effect was found but variable through time and in some instances with significant cage effects. By the end of the study, the difference in fouling load between treated and untreated nets was 14.7%. In total 72% of all free-space data points were on treated nets. The dominating fouling organisms were Enteromorpha sp. and sponges with low settlement of blue mussel and paper oysters. Passive deposition of tuna faeces contributed significantly to the cover. The largest cover of fouling was observed in April-May with a dominance of sponges in June-July. Enteromorpha sp. dominated shallow depths while sponges dominated at deeper levels. Tuna faeces were distributed independent of depth but varied with time. The results showed that fouling of fish cages consisted of both active settlement and passive deposition, the latter independent on antifouling treatment.

An in situ study of photosynthetic oxygen exchange and electron transport rate in the marine macroalga Ulva lactuca (Chlorophyta).

Direct comparisons between photosynthetic O(2) evolution rate and electron transport rate (ETR) were made in situ over 24 h using the benthic macroalga Ulva lactuca (Chlorophyta), growing and measured at a depth of 1.8 m, where the midday irradiance rose to 400-600 mumol photons m(-2) s(-1). O(2) exchange was measured with a 5-chamber data-logging apparatus and ETR with a submersible pulse amplitude modulated (PAM) fluorometer (Diving-PAM). Steady-state quantum yield ((F(m)'-F(t))/F(m)') decreased from 0.7 during the morning to 0.45 at midday, followed by some recovery in the late afternoon. At low to medium irradiances (0-300 mumol photons m(-2) s(-1)), there was a significant correlation between O(2) evolution and ETR, but at higher irradiances, ETR continued to increase steadily, while O(2) evolution tended towards an asymptote. However at high irradiance levels (600-1200 mumol photons m(-2) s(-1)) ETR was significantly lowered. Two methods of measuring ETR, based on either diel ambient light levels and fluorescence yields or rapid light curves, gave similar results at low to moderate irradiance levels. Nutrient enrichment (increases in [NO(3) (-)], [NH(4) (+)] and [HPO(4) (2-)] of 5- to 15-fold over ambient concentrations) resulted in an increase, within hours, in photosynthetic rates measured by both ETR and O(2) evolution techniques. At low irradiances, approximately 6.5 to 8.2 electrons passed through PS II during the evolution of one molecule of O(2), i.e., up to twice the theoretical minimum number of four. However, in nutrient-enriched treatments this ratio dropped to 5.1. The results indicate that PAM fluorescence can be used as a good indication of the photosynthetic rate only at low to medium irradiances.