Food web baseline of Araçá Bay, Atlantic southwestern: spatio-temporal variations in the d13C and d15N isotope values of suspended particulate matter, surface sediment organic matter and microphytobenthos
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1
Universidade de São Paulo/Instituto Oceanográfico, Departamento de Oceanografia Biológica, Brazil
To understand on the main primary carbon sources for consumers of a food web and to establish their trophic level, stable isotopic values of the baseline are required. Based on this knowledge, this work was developed with the objectives of identifying the main baseline components, and analysing the variability in time and space of d13C and d15N in the food web of Araçá Bay (23°49’S; 45°24’W). The study site is a protected low-tide terrace, one of few tide-dominated environments along the south-eastern coast of Brazil. This relatively small intertidal flat (750 m wide and 750 m long) is located on the continental side of a channel adjacent to the São Sebastião Island; it is also adjacent to the port and the oil terminal of the city of São Sebastião, and of the mouth of the submarine sewage outfall. The physical properties of water are impacted directly by the hydrodynamics of the channel, presenting a sharp seasonal change in the temperature and salinity. The present work is part of a multidisciplinary and interdisciplinary project, composed by thirteen modules with the objective of carrying out an integrated investigation to understand the functioning of a coastal system, considering physical, biological, and social processes, such as trophic interactions and flux of matter and energy, among other subjects. The project intends to be considered as a model of the study, with future applications in other similar regions. In this particular work, samples were obtained between January (austral summer) and July (austral winter) 2013, in the continental (Cont), intertidal (Intert) - mangrove, soft and rocky bottoms, and sublittoral (Subl) regions. Samples (n=193) of sixteen primary sources were collected for analysis of d13C and d15N (Suppl. Table 1). They were collected and treated according to the usually used protocol for isotope analysis, and were analysed at Davis Stable Isotope Facility, University of California. The first results for suspended organic matter (SPM), surface sediment organic matter (SSOM), and microphytobenthos are presented. For SPM, samples of water were collected in the continental region (stream), in the intertidal region (upper and intermediate zones), and along the 2-4, 8-10, and 20-24 m-deep isobaths of the sublittoral region. SPM was collected by gently filtering seawater - firstly using a 125 μm mesh net, then onto pre-combusted (450-490°C for 4 h), 47 mm, GF/F glass fiber filters (0.7 μm nominal pore size). For SSOM, sediment samples of the upper 2 cm layer were collected in the upper and intermediate zones of the intertidal region, and along the 8-10 and 20-24 m-deep isobaths of the sublittoral region. Microphytobenthos samples were collected in the same sites of the SSOM; samples from the surface of soft bottom were collected with a spatula, in the intertidal region, and from the superficial part of the van Veen grab, in the sublittoral region; collection was basically made according to Abrantes & Sheaves (2009). The filters and the samples of sediment and microphytobenthos were keeping frozen at -20°C. All samples were oven-dried (60° C for 24 h), ground to a homogenous powder with a mortar and pestle, and stored in sealed vials. Subsamples were packaged into 5 mm×8 mm tin capsules and stored over desiccant until the analysis could be performed. Samples of SSOM and microphytobenthos were acidified with 1N HCl. Stable isotope ratios are expressed in units of per mil (‰), according to Peterson & Fry (1987). A Minitab®17.0 routine was applied to perform parametric and non-parametric hypothesis tests (p-value<0.05), in order to verify time (summer and winter) and space effects (Cont, Intert, and Subl) on the mean d13C and d15N. The first results of the covariance analysis (GLM-ANCOVA), and grouping information using Tukey’s method are presented. The values range and the mean values of d13C and d15N of these producers (Suppl. Table 2) are in the range of the reported data in literature. For SPM, sampled in the three regions, d13C was higher in summer than in winter, but there was no difference among regions; on the contrary, two groups of regions were formed (Subl>Cont=Intert) for d15N, which showed no time difference. Microphytobenthos and SSOM, that were collected from the soft bottom of the intertidal and sublittoral regions, showed similar results, i.e., effect only of the region on d13C (Intert>Subl). Some mechanisms could explain the temporal and spatial shifts of SPM, SSOM, and microphytobenthos isotopes values: the time effect on the SPM d13C (>summer) is, most likely, related to biophysical processes associated to the seasonal dynamics; the region effect on the SPM d15N (Subl>Inter=Cont) could be related to the continental nitrogen load. And the region effect on the d13C SSOM and microphytobenthos (>Intert) is, probably, due to the higher values of the intertidal microphytobenthos.
Acknowledgements
Study funded by FAPESP (Fundação de Apoio a Pesquisa do Estado de São Paulo) through the project Biodiversity and Functioning of a Subtropical Coastal Ecosystem: a contribution to integrated management (FAPESP n° 2011/50317-5).
FAPESP Ph.D. scholarship (Marinella J. C. Pucci); CAPES MSc. scholarship (Lídia P.L. Arantes).
References
Abrantes, K, Sheaves M (2009). Food web structure in a near-pristine mangrove area of the Australian wet tropics. Estuar Coast Shelf Sci 82:597-607
Peterson BJ, Fry B (1987) Stable isotopes in ecosystem studies. Annu Rev Ecol Syst 18: 293−320
Keywords:
food web,
Marine primary producers,
Stable isotopes,
microphytobenthos,
suspended matter
Conference:
XIX Iberian Symposium on Marine Biology Studies, Porto, Portugal, 5 Sep - 9 Sep, 2016.
Presentation Type:
Poster Presentation
Topic:
3. OCEANOGRAPHY, MODELLING AND ECOSYSTEM DYNAMICS
Citation:
Soares
LS,
Arantes
LP and
Pucci
MC
(2016). Food web baseline of Araçá Bay, Atlantic southwestern: spatio-temporal variations in the d13C and d15N isotope values of suspended particulate matter, surface sediment organic matter and microphytobenthos.
Front. Mar. Sci.
Conference Abstract:
XIX Iberian Symposium on Marine Biology Studies.
doi: 10.3389/conf.FMARS.2016.05.00163
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Received:
27 Apr 2016;
Published Online:
03 Sep 2016.
*
Correspondence:
Prof. Lucy S Soares, Universidade de São Paulo/Instituto Oceanográfico, Departamento de Oceanografia Biológica, São Paulo, São Paulo, 05508-120, Brazil, lucyshsoares@gmail.com