Variability of nutrient regeneration rates and nutrient concentrations in surface sediments of the northern Great Barrier Reef shelf

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1 Continental Shelf Research 21 (2001) Variability of nutrient regeneration rates and nutrient concentrations in surface sediments of the northern Great Barrier Reef shelf M.J. Lourey 1, D.M. Alongi*, D.A.J. Ryan 2, M.J. Devlin 3 Australian Institute of Marine Science, PMB No. 3, Townsville M.C., Qld 4810, Australia Received 7 June 1999; accepted 6 July 2000 Abstract Sediment nutrient concentrations and fluxes were examined on the northern Great Barrier Reef (GBR) shelf to determine the extent of temporal and spatial variability. Despite a clear sediment gradient of increasing carbonate content seaward and differences in river discharge along-shelf, no significant differences in nutrient concentrations or fluxes across the sediment water interface were observed for most nutrients along- or across-shelf over time. Only interstitial P and DOC concentrations displayed significant differences along-shelf and only in the wet season. Rates of nutrient regeneration were highly variable, but contributed, on average, 11% of N and 22% of P requirements for phytoplankton production, similar to the benthic contribution for primary production on the central GBR shelf. The lack of clear patterns in the size of the sediment nutrient pools and fluxes are contrary to patterns on most other shelves, and may be partly the result of intense dilution, mixing and transport processes operating at small to large spatial scales over time. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Carbon; Nitrogen; Phosphorus; Nutrients; Sediments; Great Barrier Reef *Corresponding author. Tel.: ; fax: address: d.alongi@aims.gov.au (D.M. Alongi). 1 Present address: Institute of Antarctic and Southern Ocean Studies, University of Tasmania, P.O. Box , Hobart, Tasmania 7001, Australia 2 Present address: Atlantic Food and Horticulture Research Centre, Agriculture and Agri-Food Canada, 32 Main Street, Kentville, NS, Canada B4N 1J5 3 Present address: Great Barrier Reef Marine Park Authority, P.O. Box 1379, Townsville, Qld 4810, Australia /01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S (00)

2 146 M.J. Lourey et al. / Continental Shelf Research 21 (2001) Introduction Benthic nutrient cycles on tropical continental shelves are poorly understood mainly because such environments have been so rarely sampled. This is lamentable as continental shelves in tropical latitudes have been subjected to increasing levels of anthropogenic input (Alongi, 1998). Along the northern section of the Great Barrier Reef (GBR) shelf, most river catchments are heavily cultivated, implying that significant amounts of nitrogen- and phosphorus-based fertilisers may be leached into rivers and estuaries and transported onto the shelf (Mitchell and Furnas, 1997; Wasson, 1997). However, we are aware of only one published report on sediment nutrients on the northern GBR shelf (Eyre, 1993); indeed, nearly all benthic studies within the GBR province have been limited to either one season or one location, or both (Alongi, 1997). This study was designed to redress the lack of baseline information on sediment nutrient concentrations and rates of sediment water exchange on this portion of the GBR shelf. Such information may be needed in the near future to assess possible impacts of predicted increases in land-derived nutrient loading as a consequence of increased human encroachment along the northern Queensland coast. 2. Methods A series of stations were selected in across-shelf transects off the mouths of the Barron, Johnstone and Pascoe Rivers out to individual reefs in the northern section of the GBR shelf (Fig. 1). Sampling was conducted in alternate dry and wet seasons in , but due to constraints on time and resources, the transects were sampled only in the summer wet seasons in 1994 and Two stations were sampled on each of three transects off the Johnstone and Barron Rivers (Fig. 1). Two stations were sampled on each of two transects off the Pascoe River (Fig. 1). Inshore stations were located within 1 km of the shore. Seaward stations were located 20 km offshore, adjacent to reefs along the outer edge of the shelf. Sediment cores were obtained at each station using a modified Bouma boxcorer (0.027 m 2 area) with an average depth of penetration to 20 cm. Boxcorer samples were stored in a flowing seawater bath at ambient temperature until processed. Replicate boxcores (n=2 3) at each station were subsampled for porewater and solid-phase nutrients by inserting two aluminium cores (7 cm inner diameter) into each box to a depth >10 cm. The coring tubes contained a recessed PVC inner tube divided into 2 cm rings. The top 5 rings (10 cm) of sediment were collected by running a clean stainless-steel blade between each ring. Sediments were immediately placed into acid-washed Petri dishes for porewater extraction under a N 2 atmosphere (Alongi, 1990). Sections of bulk sediment were placed into plastic vials and frozen for later analysis of solid-phase elements. Porewater was extracted using a modified Teflon porewater extractor (Robbins and Gustinis, 1976). Porewater was squeezed through 0.4 mm Poretics TM polycarbonate membrane filters under an applied nitrogen pressure of 100 kpa until approximately 10 ml of interstitial water was collected. Samples for inorganic nutrient analysis were stored frozen in acid-washed test tubes. Samples for DOC analysis were stored at 48C in acid-washed, Teflon-capped glass vials containing 100 ml of 10% (v/v) HCl to remove carbonates.

3 M.J. Lourey et al. / Continental Shelf Research 21 (2001) Fig. 1. Location of along- and across-shelf transects on the northern Great Barrier Reef continental shelf. Upper map denotes station locations in proximity to the Pascoe River; the middle map denotes station locations in proximity to the Barron River, and the bottom map indicates stations in proximity to the Johnstone River.

4 148 M.J. Lourey et al. / Continental Shelf Research 21 (2001) Nutrient flux experiments were carried out shortly after collection of boxcores. Replicate clear glass bell jars (1 l volume) were placed into 2 3 intact, undisturbed boxcores incubated in a continuously flowing water bath to mimic natural conditions. The bell jars were pushed into the sediment to a depth of 1 2 cm. Each bell jar had two glass arms. One arm was fitted with a Teflon tube to withdraw samples with a plastic syringe. The other arm was fitted with a cap that allowed replacement water to seep in to avoid enhanced interstitial water exchange during sampling. The neck of the bell jar was fitted with a propeller and an electric stirrer motor. The chambers were stirred at a rate slow enough not to disturb the sediments. Samples were taken from each jar immediately and at 45 min intervals for 3 h. Samples for dissolved nutrients were filtered through 0.45 mm Sartorius Minisart TM filters and stored frozen until analysis. Samples for DOC were filtered through 0.4 mm Nuclepore TM filters and stored at 48C until analysis. Nutrient flux samples were analysed for dissolved inorganic nutrients (NH 4 +,NO 2 +NO 3, and PO 4 3 ) using standard automated techniques (Ryle et al., 1981; Ryle and Wellington, 1982). Total dissolved nutrient concentrations in both nutrient flux and porewater samples were determined after 8 h digestion in a UV photooxidation apparatus. Dissolved inorganic nutrient concentrations were subtracted from the total dissolved nutrient concentrations for each sample to derive DON and DOP concentrations. Solid-phase nutrients were measured in bulk sediment samples dried at 808C for 24 h and ground to a fine powder. Total organic carbon was determined on a Beckman Model 915-B Tocamaster TM total carbon analyser. Total carbon and total nitrogen were measured using a Perkin-Elmer TM Model 2400 CHN analyser. Total phosphorus was measured using a Varian TM Liberty 220 plasma emission spectrometer after perchloric/nitric acid digestion. Carbonate content was estimated by the difference between total carbon and total organic carbon concentrations multiplied by Nutrient flux rates were determined by least-squares regression. Analyses were carried out on data grouped by along- and across-shelf position, averaged across replicates. Nested ANOVA s were used to test for differences between the means of each along- and across-shelf combination. Results were compared at the 5% level of significance. A nested model was used to estimate variability between stations, years, and replicates. The model fit was a mixed linear model, incorporating the fixed effects of shelf position, season and their associated interactions (SAS Institute Inc., 1996). 3. Results and discussion In the dry season, dissolved nutrient concentrations showed no significant differences along-shelf (Figs. 2 and 3). In the wet season, only porewater DOC and TDP concentrations showed any differences with river position (Fig. 2), with wet season concentrations of porewater DOC also varying significantly with shelf position (Fig. 3). Mean porewater DOC concentrations were significantly higher in inner shelf sediments off the Pascoe River compared to sediments off the other catchments (Fig. 2). Porewater TDP concentrations were also higher off the Pascoe River in the wet season (Fig. 2). In this case, the difference was consistent for both inner and mid-shelf positions. For nutrient regeneration rates, only DOC fluxes in the dry season were significantly different along-shelf (Fig. 4) and only DON fluxes varied across-shelf, being greater on the inner shelf

5 M.J. Lourey et al. / Continental Shelf Research 21 (2001) Fig. 2. Effect of along-shelf position on depth-averaged sediment carbonate content, dissolved and solid-phase nutrient concentrations in the wet season. Vertical lines are 95% confidence limits of the mean. Light bars denote inner shelf stations; dark bars denote mid-shelf locations.

6 150 M.J. Lourey et al. / Continental Shelf Research 21 (2001) Fig. 3. Effect of shelf position and season on depth-averaged sediment carbonate content, dissolved and solid-phase nutrient concentrations. Vertical lines are 95% confidence limits of the mean. Light bars denote inner shelf stations; dark bars denote mid-shelf locations.

7 M.J. Lourey et al. / Continental Shelf Research 21 (2001) (Fig. 4). Off the Barron River, sediments released DOC in the dry season, but off the Johnstone River there was no significant DOC flux (Fig. 4). In the wet season, nutrient flux rates were not significantly different compared to the dry season or along- or across-shelf. The statistical analyses revealed significant river X shelf interactions for NO 2,NO 3 and DOC fluxes, reflecting highest N fluxes (up to 20 mmol m 2 h 1 ) mid-shelf off the Pascoe River and highest DOC flux (up to 40 mmol m 2 h 1 ) inner shelf off the Johnstone River. For solid-phase elements, there were significantly higher carbonate and phosphorus concentrations in mid-shelf sediments compared to inner shelf sediments in both the wet and dry seasons (Fig. 2). There were no significant temporal or spatial patterns for the other solidphase elements (Figs. 2 and 3). Porewater nutrient concentrations and nutrient fluxes exhibited variability at the level of replicates (range for nutrient concentrations: %, range for nutrient fluxes: % of total variability) and from year-to-year (nutrient concentrations: %, nutrient fluxes: % of total variability), with little or no variability with shelf position (nutrient concentrations: 0%; nutrient fluxes: 0 9.2% of total variability). The trend for solid-phase concentrations was the opposite, with most variability between stations (range: % of total variability) and little variability among replicates (range: %). Year-to-year variability was greater for total N (33.9%) and TOC (19.0%) than for carbonate (2.3%) and total P (9.6 % of total variability). Despite clear differences in along- and across-shelf patterns of primary production (Furnas and Mitchell, 1997), river discharge (Moss et al., 1992) and carbonate content (Belperio, 1983), few, if any, clear temporal and spatial differences in nutrient concentrations and fluxes from surface sediments were found. Within- and between-station variability was high for some elements, so no significant broad-scale trends could be isolated. These results are contrary to temporal and spatial patterns of sediment nutrient dynamics on most other continental shelves, where benthic nutrient regeneration and standing stocks are driven mainly by seasonal temperature changes, sediment granulometry, and patterns of primary production (Alongi, 1998). On the GBR shelf, seasonal temperature changes are small compared to shelves of higher latitude, and primary production is vested mainly in picoplankton communities that are abundant and productive year-round (Liston et al., 1992; Furnas and Mitchell, 1997). The GBR shelf does, however, have a clear gradient from terrigenous sediments inshore to carbonate sediments at the shelf edge (Belperio, 1983; Alongi, 1989). The significant differences that we found are not easily explained. Porewater DOC and phosphorus levels were highest inshore adjacent to the relatively pristine Pascoe River. This is contrary to the expectation that agricultural runoff would have a demonstrable impact on coastal sediment chemistry. The nutrient concentrations and fluxes measured at our stations were not elevated compared with nutrients measured in pristine sediments on other areas of the GBR shelf (Hansen et al., 1987; Ullman and Sandstrom, 1987; Alongi, 1989, 1990; Eyre, 1993). A number of factors may account for the lack of clear patterns of sediment nutrients in the northern GBR. First, with the exception of flood events caused by cyclones, material discharged from rivers is retained close to the coast and transported northwards by currents generated by southeasterly trade winds (Wolanski, 1994). This is also supported by evidence from lignin markers (Susic and Alongi, 1997), trace metals (Alongi et al., 1993) and measurements of bulk sediment composition (Belperio, 1983) showing restriction of terrestrial material to the coastal

8 152 M.J. Lourey et al. / Continental Shelf Research 21 (2001) Fig. 4. Effect of along-shelf position on nutrient regeneration rates across the sediment water interface in the dry season. Vertical lines are 95% confidence limits of the mean. Black points are inner shelf locations; open points are midshelf locations.

9 M.J. Lourey et al. / Continental Shelf Research 21 (2001) zone. The across-shelf carbonate gradient suggests that inner and mid-shelf sediments are distinct, but no particular along-shore patterns of nutrient concentration or flux were detected in this study. Nutrients introduced onto the shelf from rivers may be diluted in the coastal zone, and processed rapidly on the inner shelf, and/or transported in an along-shore direction, causing spatial patchiness to be minimized. Within-station variability was high, suggesting that local physical, biological and chemical processes had a marked effect on the way estuarine material is distributed onto the shelf and reworked. Second, resuspension caused by tidal scouring and cyclones, and sediment mixing by bioturbation, may play a role in ameliorating spatial and temporal changes in sediment nutrient concentrations and fluxes. On the central GBR shelf, it is known that tidal scouring, cyclonic disturbances, and prawn trawling all play a role in periodically resuspending large areas of the seabed (Torgersen and Chivas, 1985; Gagan et al., 1988; Alongi, 1989). The GBR shelf is shallow and prone to intense remixing of surface sediments during cyclones and storms, and prolonged periods of strong winds. The intensity of bioturbation is unknown for our stations, but radiochemical data from intertidal and subtidal deposits further south indicate very variable mixed layer thicknesses from 0 to 30 cm depth, suggesting high spatial heterogeneity of surface sediments on this shelf (Torgersen and Chivas, 1985). In other coastal ecosystems, sediment resuspension results in a dampening of seasonal cycles in pelagic and benthic microbial activity and nutrient recycling, even in temperate systems strongly driven by seasonal changes in temperature and light intensity (Alongi, 1998). A similar dampening by resuspension events may occur on the northern GBR shelf. Nutrients transported onto the GBR shelf are rapidly taken up by pelagic primary producers (Furnas et al., 1995, 1997). This may mask any clear temporal or spatial differences in sediment nutrient dynamics. Extensive hydrographic surveys indicate high variability in phytoplankton biomass and productivity with maximum plankton activity and standing stocks in coastal and lagoonal waters (Liston et al., 1992; Furnas et al., 1995, 1997). The slow rates of nutrient flux measured in this study suggest that nutrients released across the sediment water interface support only a small proportion of shelf phytoplankton production. Primary production in this area averages 680 mg C m 2 d 1 (Furnas et al., 1995). Assuming Redfield stoichiometry (Redfield et al., 1963) and using the primary production data of Furnas et al. (1995), the N and P requirements for shelf phytoplankton production are estimated at 8.53 and 0.53 mmol m 2 d 1, respectively. Averaging stations and seasons, benthic nutrient fluxes contribute 11% of N and 22% of P requirements for phytoplankton production on this portion of the shelf. These estimates compare favorably with earlier studies (Alongi, 1989, 1990) that found that central GBR sediments provided 6 13% of N and 9 24% of P requirements for phytoplankton production. The bulk of N and P required for phytoplankton production on the GBR shelf likely originates from microbial recycling in the water column (Furnas et al., 1995). Acknowledgements We thank the masters and crew of RV The Harry Messel, P. Christoffersen, I. Zagorskis, N. Johnston, S. Windsor, F. Tirendi, S. Boyle, and C. Payn for field and lab assistance. Contribution No from the Australian Institute of Marine Science.

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