Importance of intertidal sediment processes and porewater exchange on the water column biogeochemistry in a pristine mangrove creek (Ras Dege, Tanzania)

Abstract

Abstract. We sampled a tidal creek (Ras Dege, Tanzania) during a 24-h cycle to document the variations in a suite of creek water column characteristics and to determine the relative influence of tidal and biological driving forces. Since the creek has no upstream freshwater inputs, highest salinity was observed at low tide, due to evaporation effects and porewater seepage. Total suspended matter (TSM) and particulate organic carbon (POC) showed distinct maxima at periods of highest water flow, indicating that erosion of surface sediments and/or resuspension of bottom sediments were an important source of particulate material. Dissolved organic carbon (DOC), in contrast, varied in phase with water height and was highest at low tide. Stable isotope data of POC and DOC displayed large variations in both pools, and similarly followed the variations in water height. Although the variation of δ 13 C DOC (−23.8 to −13.8‰) was higher than that of δ 13 C POC (−26.2 to −20.5‰), due to the different endmember pool sizes, the δ 13 C signatures of both pools differed only slightly at low tide, but up to 9‰ at high tide. Thus, at low tide both DOC and POC originated from mangrove production. At high tide, however, the DOC pool had signatures consistent with a high contribution of seagrass-derived material, whereas the POC pool was dominated by marine phytoplankton. Daily variations in CH 4 , and partial pressure of CO 2 (pCO 2 ) were similarly governed by tidal influence and were up to 7-and 10-fold higher at low tide, which stresses the importance of exchange of porewater and diffusive fluxes to the water column. When assuming that the high dissolved Correspondence to: S. Bouillon (steven.bouillon@vub.ac.be) inorganic carbon (DIC) levels in the upper parts of the creek (i.e. at low tide) are due to inputs from mineralization, δ 13 C data on DIC indicate that the organic matter source for mineralization had a signature of −22.4‰. Hence, imported POC and DOC from the marine environment contributes strongly to overall mineralization within the mangrove system. Our data demonstrate how biogeochemical processes in the intertidal zone appear to be prominent drivers of element concentrations and isotope signatures in the water column, and how pathways of dissolved and particulate matter transport are fundamentally different.

Short-Term Dissolved Organic Carbon Dynamics Reflect Tidal, Water Management, and Precipitation Patterns in a Subtropical Estuary

“…The location where the sonde was deployed is immediately adjacent to mangrove sediments perforated with crab burrows, a feature shown to enhance hydraulic connectivity of mangrove sediments to the water column [up to 20% in some systems, Stieglitz et al (2013)]. Reduced dissolved oxygen (DO) levels, often associated with porewater hypoxia in mangrove ecosystems (Bouillon et al, 2007), were generally present during FDOM spikes supporting the hypothesis of porewater contributions. However, these patterns were somewhat variable, with examples in Figure 4 showing consistently depleted DO concurrent with FDOM spikes during T2 but not T1.…”

Dissolved carbon biogeochemistry and export in mangrove-dominated rivers of the Florida Everglades

“…In fact, several studies have shown that the lateral advective transport of interstitial waters through tidal pumping represents a major carbon export pathway from mangroves into adjacent waters, both for DIC (Koné and Borges, 2008;Miyajima et al, 2009; Published by Copernicus Publications on behalf of the European Geosciences Union. 2544 D. T. Ho et al: Dissolved carbon export from mangroves Maher et al, 2013) and dissolved organic carbon (DOC) (Dittmar and Lara, 2001;Bouillon et al, 2007c). However, to date, lateral mangrove-derived aquatic carbon fluxes (as a proportion of overall forest carbon mass balance) have only been estimated for short time periods and over limited spatial (e.g., plot) scales (e.g., Troxler et al, 2015).…”

Statistical Perspective and Pollution Indicator in Mengkabong Mangrove Sediment Sabah

“…Its composition reflects the sediment chemistry (Rand, 1995). Bouillon et al (2007) showed that exchange of pore-water, combined with the physical conditions related to the tidal cycle, were important in explaining the sediment chemistry at high and low tide. Sediment pore-water represents the mediator fluid in the exchange of the components between sediment and water.…”