Spatial Gradients of Carbon, Alkalinity, and Radium Isotopes in Brazilian Mangrove Seascapes

Sampling was conducted in two mangrove seascapes in Brazil covering the intertidal porewater-mangrove creek-coastal bay-shelf continuum. The first site is situated in a tropical mesotidal region (23°18'06.2S 44°38'53.6W) near Paraty city (Rio de Janeiro). The mangrove catchment area (260,112 m2) is centered in a pristine setting connected to the southeast Brazil shelf through the oligotrophic and shallow (~5m) Mamanguá bay. The second seascape is in a subtropical microtidal region (27°38'55.6S 48°33'11.2W), located in Florianópolis city in the State of Santa Catarina. The mangrove creek (146,065 m2) is surrounded by urban areas and potentially impacted by sewage and agricultural runoff discharging in the Santa Catarina Island Bay. Mangroves at both sites have a mixed semidiurnal tidal pattern.A small vessel was moored in the mouth of the mangrove creeks for time series sampling at neap and spring tides to serve as a temporary sampling station. Surface water samples (0.5 m depth) were collected each hour for 30 hours using a peristaltic pump (Solinst, Model 410). Samples for carbon and alkalinity were collected using sample-rinsed 60 ml polypropylene syringes and pre-combusted (450°C 4h) 0.7 µm Whatman GF/F filters.Water samples for dissolved inorganic carbon (DIC) and its stable isotope composition (δ13C – DIC) were stored in gas-tight 12 ml evacuated exetainer with rubber septum (Labco UK), poisoned with 20 µL of a saturated solution of HgCl2 and stored at 4°C in the dark. DIC samples were analyzed using a AS-C5 (multi-port version, Apollo SciTech, LLC) with an infrared CO2 detector (LI-850, LI-COR, USA). δ13C – DIC samples were analyzed by EA-IRMS, an elemental analyzer (Carlo Erba 1110, CE Instruments, Italy) coupled to a Thermo Scientific Delta V Advantage isotope-ratio mass spectrometer. δ13C – DIC measurements were calibrated with certified reference materials LSVEC and NBS-19.Total alkalinity (TA) samples were stored in 50 ml Falcon tubes at 4°C in the dark until analysis using potentiometric Gran titration with 0.01 M HCl in an 888 Titrando (Tiamo software, Metrohm AG). DIC and TA samples were calibrated using measurements of certified reference material (CRM) Batch #197 from the Scripps Institution of Oceanography (UCSD) provided by Andrew Dickson.DOC samples were collected in 60 ml Thermo Scientific Nalgene HDPE bottles acidified with 600 µL of concentrated H3PO4 and stored at 4°C in the dark. DOC was analyzed using a Shimadzu 5000. Particulate material was collected by filtering 0.1 – 1L of seawater through pre-combusted (450°C 4h) 0.7 µm Whatman GF/F filters and stored at -20°C until analysis. Inorganic carbon was removed by exposing the filters to concentrated HCl fume for 24 h. POC was determined using a FLASH 2000 elemental analyzer (Thermo Scientific). The analytical control was performed by sampling replicates (Coefficient of Variation < 10%) and certified standards (Elemental Microanalysis Protein Standard).Water samples for short-lived radium isotopes were collected with submergible bilge pump into 2-80L barrels and filtered through 25g of Mn fibers at < 1 L min-1. The fibers were rinsed with radium-free water, partially dried and placed in Radium Delayed Coincidence Counters (RaDeCC) to measure 224Ra and 223Ra. Radium samples were first analyzed between 1-3 days after sampling and again after one month to account for 224Ra from 228Th decay.Salinity and temperature (Solinst Levelogger 5), dissolved oxygen (DO, PME miniDOT) and pH (Onset HOBO pH logger) probes were attached to the vessel at 0.5 m depth to record every minute. The probe data were smoothed to 1 hour time steps to enable comparisons with discrete water samples.Porewater samples were collected using a peristaltic pump during ebb tide in both mangroves towards the end of neap (N = 12) and spring (N = 12) tides. In the mesotidal mangrove, porewater was sampled from seeping water from crab burrows, which integrates the creek intertidal sediment signature. In the microtidal mangrove, bores were dug in the mud up to 50 cm depth and purged for several minutes before sampling after porewater recharging at least three times. Transects extending from the mangroves into the bays and across the continental shelves were conducted to measure the aforementioned variables in surface waters.