MARINE ECOLOGY PROGRESS SERIES Vol. 186: 119-126,1999 Published September 17 Mar Ecol Prog Ser Nitrogen fixation by Trichodesmium spp. in the Central and Northern Great Barrier Reef Lagoon: relative importance of the fixed-nitrogen load 'Low Isles Research Station and Department of Chemical Engineering, University of Queensland, Brisbane 40'12, Australia 'Centre for Microscopy and Microanalysis, University of Queensland, Brisbane 4072, Australia ABSTRACT: Studies in the Great Barrier Reef Lagoon (GBRL) show that Trichodesmium (OsciUatoria) spp. fix significant amounts of atmospheric nitrogen and that the loads of 'new' nitrogen introduced by 7'richodesm.um spp. are at least of the same order as those entering via riverine discharge. The ele- vated growth of other genera of phytoplankton and hence eutrophication in some sections of the GBRL could now be largely driven by the in situ production of this 'new' nitrogen. 7'richodesmium is more prolific in the Central GBRL than in the Northern GBRL and there is evidence that the concentrations of Trichodesmiumspp. have increased since the 1928-29 Great Barrier Reef Expedition to Low Isles. It is hypothesised that this increase has resulted from increases in river borne nutrients that would pro- mote N fixation (e.g. phosphorus, iron and dissolved organic matter). It is estimated that the loads of such nutrients have increased several fold since the development of the coastal catchments of Queens- land. KEY WORDS: Trichodesmium - Nitrogen fixation - Eutrophication - Great Barrier Reef INTRODUCTION ern Sargasso Sea show an average N fixation rate of 0.069 pg N cell-' h-'. They conclude that N fixation is The Great Barrier Reef Lagoon (GBRL) in this study the main source of N for T. thiebautii, the most abun- refers to the shallow (c40 m) water body bounded by dant of the 3 Trichodesmium species identified in their the mainland and the outer reef matrix of the Great Caribbean studies. T. thiebautiialso occurs in the GBRL Barrier Reef (GBR) (Fig. 1). Trichodesmium spp. often but T. erythraeum is often dominant (Revelante & dominate the microplankton (>20 p)community in Gilrnartin 1982). Bryceson & Fay (1981)have measured the Central (off Townsville) and Northern (off Port Dou- the acetylene reduction rate for T. erythraeum collected glas/Low Isles) GBRL (Marshal1 1933, Revelante & in the coastal waters off Tanzania (East Africa). From Gilmartin 1982, Elmetri 1993). Tnchodesmium blooms their results the mean value of the N fixation rate per in the region have been implicated in direct smothering cell is, assuming the conversion factor of 4:l and 100 of corals and the promotion of the bioavailability of cells per trichome (filament) (Capone 1993, Carpenter heavy metals (Endean 1976, Jones 1987,1992).Various & Capone 1992), 0.095 pg N cell-' h-', which is in quite workers have shown that Tnchodesmium spp. fix good agreement with the above-mentioned values for atmospheric nitrogen (Goering et al. 1966, Carpenter & T. thiebautii. Higher values have been recorded for McCarthy 1975). Carpenter & Price (1977), using the blooms in some coastal waters; Carpenter & Capone acetylene reduction method (a sensitive measure of (1992) suggest that a typical value is 1.5 pg N cell-' h-'. nitrogenase activity and hence N fixation), observed Results from acetylene reduction studies with Tri- average N fixation rates of 0.077 pg N cell-' h-' in the chodesmium erythraeum in the Northern and Central Caribbean Sea and 0.033 pg N ceV' h-' in the Sargasso GBRL equate to (assuming a 4:1 conversion ratio, Sea. Data of Carpenter & McCarthy (1975)for the west- Capone 1993) a mean N fixation rate of 0.08 pg N cell-' h-' (Bell et al. 1993). This result is in good agreement with the results from several of the above-mentioned O Inter-Research 1999 Resale of full article not permitted 120 Mar Ecol Prog Ser 186: 119-126, 1999 coast catchments and Great Barrier Reef (GBR) Lagoon, Cen- tral GBR sampling sta- tions I, 11, 111 used by Revelante & Gilmartin (1982) and Northern GBR sampling sta- m. Extent of East Coast tions 1, 2, 3, 4, 3ME Catchments of Queensland and BR used in this study are shown studies. The available data on the standing crop of pound/binocular microscope (X 100 magnification). Tnchodesmium in the GBRL are quite limited but these This filtering method provides equivalent counts to the data do suggest that relatively high concentrations of Utermohl settling method (Elmetri 1993) but is rela- Trichodesmium spp. occur regularly, and hence Tri- tively quick in comparison with that method and does chodesmium spp. have the potential to introduce large not require the use of an inverted microscope. amounts of 'new' nitrogen into the GBRL. The work Variation in standing crop of Trichodesmium in reported below provides additional quantitative infor- Cleveland Bay (off Townsville). Stn I in Cleveland Bay mation on the rates of N fixation of Trichodesmium in (off Townsville) was occupied at approximately weekly the Northern GBRL and standing crop of Trichodes- intervals from September 1993 to January 1994. Water miurn spp. in the Northern and Central GBRL. These samples were collected from 0.5 m depth using a l0 1 data are used to estimate the loads of 'new' nitrogen Niskin bottle. Several litres were filtered (0.45 pm pore introduced to the Northern and Central GBRL by Tn- size glass fibre filter) on-site for subsequent chloro- chodesmium. phyll a analysis. The filters were stored in a cooler and frozen within 1 h of collection. Two 250 m1 samples were preserved on-site with Lugol's solution and were MATERIALS AND METHODS later analysed for Trichodesmium spp. using the above-mentioned filter method and for the other prin- Variation in standing crop of Trichodesmium at cipal genera in the microphytoplankton using Uter- Low Isles. Trichodesmium concentrations were deter- mohl's inverted microscope method. mined in surface samples (0.5 m) collected every 1 to Nitrogen fixation studies-Low Isles. Four series of 2 wk during 1992-93 at a sampling station located 3 experiments were completed. Series 1 and 2 were of 1d miles east (Stn 3ME) off Low Isles, the same station duration only. For Series 1 and 2, water samples were used by the 1928-29 Expedition (Marshal1 1933, Orr collected from 1 m depth with a depth sampler from a 1933). In addition to the regular sampling, 2 bloom station located 1 km north of Low Isles. Series 3 was un- events (30-31 October 1995 and 10 October 1997) were dertaken over a 4 d period during a prolonged bloom sampled to determine the cross-shelf and depth varia- event; samples were collected as for Series 1 and 2, tion of the Trichodesmium standing crop. Depth sam- from the same location. Series 4 samples were collected ples were collected at 4 sampling stations (Stns 1 to 4) from the Anchorage at Low Isles, on 2 days immedi- on a transect between Port Douglas and Low Isles, and ately following Series 3. All water samples were col- also at Stn 3ME and at Batt Reef (Stn BR), using a 10 1 lected at approximately 09:OO h and the incubations Niskin bottle. Trichodesmium concentrations were were generally conducted between 10:OO and 14:00 h. determined by filtering a preserved (Lugol's solution) The acetylene reduction method was used to esti- 200 m1 sample through a graduated membrane filter mate the N fixation rates (Stewart & Alexander 1971, (0.45 pm pore size) and counting the trichomes (fila- Mague et al. 1974, Carpenter & McCarthy 1975, Car- ments) retained on the filter using a normal com- penter & Price 1977, Bryceson & Fay 1981, Capone Bell et al.: Nitrogen fixation by Trichodesmium spp. 121 1993).The samples were concentrated by gently filtering off most of the water through a 20 pm glass fibre filter. Colonies, both radial (R) and longitudinal (L), and individual trichomes (T) from the concentrate were gently pipetted from the concentrate and washed in filtered (0.45 pm) seawater and then transferred to filtered seawater contained in wide- mouth serum bottles (approximately 80 %) full. Each bottle was sealed with a long- skirted red-silicone rubber serum stop- Sampling Date (month) per. Scrubbed acetylene, which had been Fig. 2. Comparison of seasonal variation of Trichodesmium spp. (trichomes freshly prepared on-site from calcium I-') at Stn 3ME near Low Isles in 1928-29 (Marshall 1933) and 1992-93 carbide, was injected into each bottle using a gas-tight syringe to give an initial concentration of acetylene of at least 15 %. A gas phase sample was extracted from each bottle RESULTS at zero time using a gas-tight syringe and was analysed immediately for ethylene concentration using Standing crop of Trichodesmium at Low Isles and in a Photovac 10s plus portable GC fitted with a photo- Cleveland Bay ionisation detector (PID). The bottles were incubated indoors at 25 * 1.5"C for 2 to 3 h in natural shaded sun- The seasonal variation of Trichodesmium spp. at Stn light. Gas samples were then extracted and analysed 3ME near to Low Isles during 1992-93 was compared for ethylene concentration on the GC. The gas phases with the corresponding data collected by Marshal1 of several control blanks, i.e. bottles containing filtered (1933) in 1928-29. This comparison suggests that sea water with added acetylene but with no added Tri- there has been a significant increase in the standing chodesmium, were also analysed at zero time and after crop of Trichodesmium spp. since 1928-29 (Fig. 2). the incubation period in order to determine the back- However, the difference is probably not as great as is ground ethylene concentration in the absence of any indicated.