Continental Shelf Research 21 (2001) 145–155
Variability of nutrient regeneration rates and nutrient
concentrations in surface sediments of the northern
Great Barrier Reef shelf
M.J. Lourey1, D.M. Alongi*, D.A.J. Ryan2, M.J. Devlin3
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.: +61-7-4753-4211; fax: +61-7-4772-5852.
E-mail address: [email protected] (D.M. Alongi).
1
Present address: Institute of Antarctic and Southern Ocean Studies, University of Tasmania, P.O. Box 252-77,
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.
0278-4343/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 2 7 8 - 4 3 4 3 ( 0 0 ) 0 0 0 8 4 - 4
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M.J. Lourey et al. / Continental Shelf Research 21 (2001) 145–155
1. 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 1992–1993, but due to
constraints on time and resources, the transects were sampled only in the summer wet seasons in
1994 and 1995. 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 m2 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 N2 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 PoreticsTM 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.
M.J. Lourey et al. / Continental Shelf Research 21 (2001) 145–155
147
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.
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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 MinisartTM filters and stored frozen until analysis. Samples for DOC were
filtered through 0.4 mm NucleporeTM filters and stored at 48C until analysis.
ÿ
ÿ
Nutrient flux samples were analysed for dissolved inorganic nutrients (NH+
4 , NO2 + NO3 , and
3ÿ
PO4 ) 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
TocamasterTM total carbon analyser. Total carbon and total nitrogen were measured using a
Perkin-ElmerTM Model 2400 CHN analyser. Total phosphorus was measured using a VarianTM
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 8.33.
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
M.J. Lourey et al. / Continental Shelf Research 21 (2001) 145–155
149
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.
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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.
M.J. Lourey et al. / Continental Shelf Research 21 (2001) 145–155
151
(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
ÿ
revealed significant river X shelf interactions for NOÿ
2 , NO3 and DOC fluxes, reflecting highest N
ÿ2 ÿ1
fluxes (up to 20 mmol m h ) mid-shelf off the Pascoe River and highest DOC flux (up to
40 mmol mÿ2 hÿ1) 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: 31.2–51.7%, range for nutrient fluxes: 19.6–71.1%
of total variability) and from year-to-year (nutrient concentrations: 48.3–68.8%, nutrient fluxes:
27.8–80.4% 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: 57.1–95.8% of
total variability) and little variability among replicates (range: 1.9–9.0%). 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
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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.
M.J. Lourey et al. / Continental Shelf Research 21 (2001) 145–155
153
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 mÿ2 dÿ1 (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 mÿ2 dÿ1, 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. 1023 from the Australian Institute of Marine Science.
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