No abstract

Ecological applications of stable isotope analysis rely on different producers having distinct isotopic ratios to trace energy and nutrient transfer to consumers. Carbon (C) and nitrogen (N) are the usual elements analysed. We tested the hypothesis that producers unable to be separated using C and N would be separated by sulphur (S), by reviewing estuarine and marine food web studies using all three elements (total of 836 pairwise comparisons between producers). S had a wider range of values across all producers than C and N (S: 34.4, C: 23.3, N: 18.7 per thousand ), and a higher mean difference among producers (S: 9.3, C: 6.5, N: 3.3 per thousand ). We varied from 1 to 10 per thousand the distance producers must be apart to be considered separate. For each of these gap distances, S-separated producers tied on C and N in 40% or more of cases. Comparing the three elements individually, S had fewer tied pairs of producers for any gap distance than C or N. However, S also has higher within-producer variability. Statistical tests on simulated data showed that this higher variability caused S to be less effective than C for analysing differences among mean producer values, yet mixing models showed that S had the smallest confidence intervals around mean estimates of source contributions to consumers. We also examined the spatial and temporal scales over which S isotope signatures of the saltmarsh plant Spartina alterniflora varied. Differences between samples taken within tens of metres were smallest, but between samples hundreds of metres apart were as different as samples thousands of kilometres apart. The time between samples being taken did not influence S signatures. Overall, the use of S is recommended because it has a high probability of distinguishing the contribution of different producers to aquatic food webs. When two elements are employed, the combination of S and C separates more producers than any other combination.

Denitrification, N-fixation, and dissolved inorganic and organic fluxes of nitrogen (N) and phosphorus (P) were measured in each of the major benthic habitat types of a shallow oligotrophic subtropical coastal system, and N and P budgets were constructed to quantify the importance of each habitat to N and P cycling in the whole ecosystem. The productivity/respiration (p/r) ratio (trophic status) of the habitats was an important control on the rates, direction (uptake, efflux) and composition (dissolved inorganic N (DIN), dissolved organic N (DON), N 2 ) of N fluxes across the sediment-water interface, with an efflux below p/r = 1.5 and an uptake above p/ r = 1.5. The Zostera Seagrass Community was the most important habitat for N loss via net N 2 effluxes (denitrification; 48%). Denitrification rates in seagrass were higher than those previously measured in temperate regions, most likely due to greater availability of NH 4? for coupled nitrification-denitrification. Yabby Shoals (sub-tidal shoals inhabited by burrowing shrimp, Trypaea australiensis) accounted for the second largest loss of N via denitrification, the largest recycling of DIN and dissolved inorganic P (DIP; statistically significant only during the dark in summer) across the sediment-water interface and the second largest uptake of DON (statistically significant only in summer). This study highlighted that shallow subtropical coastal systems have a complex mosaic of benthic habitats and that some less 'iconic' habitats (i.e. non-seagrass) also make an important functional contribution that controls the flow of N and P through the whole ecosystem.

The composition and production of carbohydrates (mannose, rhamnose, fucose, galactose, glucose, and xylose) and their transfer among sediment compartments (microphytobenthos [MPB], bacteria, and detritus) was investigated through in situ labeling with 13 C-bicarbonate. After 60 h, 13 C was found in all sediment compartments, demonstrating rapid transfer of fixed carbon from autotrophs to heterotrophs. Carbohydrates were a major carbon reservoir, accounting for 30% (day 0) to 15% (day 30) of the 13 C within sediments, and probably played a role in this transfer. Carbohydrate fractions were highly reactive (65-87%), less reactive (7-18%), and nonreactive (6-23%) over the experimental period. The rate of loss of the less reactive fraction (0.01-0.05 d 21 ) was at least an order of magnitude lower than that for the highly reactive fraction (0.8-4.4 d 21 ). Patterns of diagenesis estimated from label uptake and loss matched the carbohydrate composition observed in the sediment (glucose . galactose . rhamnose . fucose . xylose . mannose) and were similar to patterns reported previously. C : N ratios and d 13 C of sediment organic matter indicated an algal origin (MPB and phytoplankton). Although carbon was rapidly processed, loss from sediments was not immediate, and there was evidence of recycling into MPB and bacteria. Rapid transfer of carbon to and from carbohydrates has been found in various environments, including temperate, muddy, and intertidal sediments, and this study demonstrates the important role of carbohydrates in supporting heterotrophic production over extended periods (. 30 d) in subtropical shallow subtidal sands.

Microphytobenthos (MPB) in photic sediments are highly productive but the fate of this production remains uncertain. Over 33 d, tracing of 13 C from added bicarbonate in subtropical shallow subtidal sand showed rapid transfer of MPB-derived carbon to deeper sediment; below 2 cm (31% within 60 h) and 5 cm (18%). Despite their high turnover (5.5 d) and only representing , 8% of sediment organic carbon, MPB represented up to 35% of the 13 C retained in sediments, demonstrating substantial carbon recycling. Carbon was rapidly transferred to heterotrophs, but their contribution to sediment 13 C was similar to their biomass contribution (, 0.1% to 3.8%), with the exception of the foraminifera Cellanthus craticulatus, which accounted for up to 33% of the 13 C within sediment. There was little loss of MPB-derived carbon via dissolved organic carbon (DOC) effluxes (3%) or resuspension (minimal). Respiration was the major loss pathway (63%), reflecting the high microbial biomass typical of lower latitudes. Given that MPB take up dissolved inorganic carbon (DIC) from overlying water, and the carbon they fix is released as DIC, MPB in subtropical sands are unlikely to substantially alter the form of carbon transported offshore (i.e., there is no conversion to DOC), but processing within the sediment may alter its d 13 C value. Given that 31% of fixed carbon remained in sediments after 33 d, subtropical sands may act as a carbon sink, thereby affecting the quantity of carbon transported offshore.