Distribution and dynamics of particulate organic matter in Indian mangroves during dry period.

The distribution and possible sources of particulate organic carbon (POC) and particulate nitrogen (PN) in seven mangroves ecosystems along the east and west coast of India were examined, to understand their contribution to coastal biogeochemistry. Suspended particulate matter (SPM) concentration in mangrove waters were about ~ 1.6-fold higher in west coast (Gulf of Kachchh (GOK), Mandovi-Zuari (MA-ZU) and Karwar-Kumta (KR-KU)], whereas the mean POC content in SPM along east coast [Sundarbans (SUN), Bhitarkanika (BHK), Coringa (COR) and Pichavaram-Muthupet (PI-MU)] was nearly two times higher than the west coast (1.97 ± 0.91% and 1.06 ± 0.29%), respectively. The results indicated that the influence of the land-based contaminants on the water quality parameters (dissolved oxygen, pH, salinity, nutrients and chlorophyll-a, etc.), which primarily regulated the distribution and transformation of organic carbon in these mangrove waters. Among the studied systems, an extremely high DOC/POC ratio (5.72 ± 1.64) with low pH and DO in COR waters clearly indicated the labile nature of the organic matter influenced by anthropogenic stress. Strong correlation between POC and PN indicated a similar origin in particulate organic matter. The ratios of POC/PN and POC/Chl-a showed significant spatial variation ranging from 5.5 to 18.7 and 126 to 1057, respectively. The results indicated that significant fraction of in-situ primary production contributed to particulate organic matter (POM) pool in all Indian mangrove waters except the GOK and the SUN waters, where sediment resuspension and mangrove derived organic matter were the dominant POM sources.

Assessment of bioavailable nitrogen and phosphorus content in the sediments of Indian mangroves.

Efficient nutrient cycling and adequate sediment bioavailable nutrient supply are considered to be the two most important factors regulating the high productivity and subsequent carbon sequestration by mangrove ecosystems. We assessed spatial variability and the possible regulating factors of sediment bioavailable nutrients (nitrogen (N) and phosphorus (P)) and surface water-dissolved nutrients (N, P and silicate (Si)) in the five ecologically important mangrove ecosystems along the east and west coast of India during dry season. Higher bioavailable nitrogen concentrations in the sediments were recorded in Coringa mangroves (36.27 ± 14.7 µg g-1) and Bhitarkanika (18.54 ± 5.9 µg g-1) mangroves in the east coast followed by Karnataka (15.51 ± 8.26 µg g-1), Goa, (10.18 ± 9.96 µg g-1) and Kerala (6.36 ± 5.05 µg g-1) mangroves in the west coast. The dissolved inorganic nutrients in the mangrove waters ranged between 5.1 and 220.9 µmol l-1 for N and 0.07 and 3.9 µmol l-1 for P. These results indicated that terrestrial inputs, in situ remineralization and prevalent anoxic conditions regulated sediment nutrient content in these ecosystems, whereas the higher ammonium in the sediments was attributed to the greater nutrient adsorption by finer particles. The stoichiometry of the bioavailable nutrients (N, P) in the mangrove sediments deviated drastically from the Redfield ratio, and strong P limitation was recorded in most of the ecosystems. The results highlighted the potential role of sediment particle size and physiochemical (salinity and pH) properties in regulating bioavailable nutrient dynamics in mangrove sediments.

Seagrass and macrophyte mediated CO2 and CH4 dynamics in shallow coastal waters.

Seagrass meadows are among the most important coastal/ marine ecosystems for long-term carbon storage and conditioning of coastal waters. A combined air-water flux of CO2 and CH4 from the seagrass meadows was studied for the first time from Asia's largest brackish-water lagoon, Chilika, India. Ecosystem-based comparisons were carried out during two hydrologically different conditions of dry and wet seasons in the seagrass dominated southern sector (SS); macrophyte-dominated northern sector (NS); the largely un-vegetated central sector (CS) and the tidally active outer channel (OC) of the lagoon. The mean fluxes of CO2 from SS, NS, CS and OC were 9.8, 146.6, 48.4 and 33.0mM m-2d-1, and that of CH4 were 0.12, 0.11, 0.05 and 0.07mM m-2d-1, respectively. The net emissions (in terms of CO2 equivalents), considering the global warming potential of CO2 (GWP: 1) and CH4 (GWP: 28) from seagrass meadows were over 14 times lower compared to the macrophyte-dominated sector of the lagoon. Contrasting emissivity characteristics of CO2 and CH4 were observed between macrophytes and seagrass, with the former being a persistent source of CO2. It is inferred that although seagrass meadows act as a weak source of CH4, they could be effective sinks of CO2 if land-based pollution sources are minimized.

Seagrass metabolism and carbon dynamics in a tropical coastal embayment.

Net ecosystem metabolism and subsequent changes in environmental variables were studied seasonally in the seagrass-dominated Palk Bay, located along the southeast coast of India. The results showed that although the water column was typically net heterotrophic, the ecosystem as a whole displayed autotrophic characteristics. The mean net community production from the seagrass meadows was 99.31 ± 45.13 mM C m-2 d-1, while the P/R ratio varied between 1.49 and 1.56. Oxygen produced through in situ photosynthesis, exhibited higher dependence over dissolved CO2 and available light. Apportionment of carbon stores in biomass indicated that nearly three-fourths were available belowground compared to aboveground. However, the sediment horizon accumulated nearly 40 times more carbon than live biomass. The carbon storage capacities of the sediments and seagrass biomass were comparable with the global mean for seagrass meadows. The results of this study highlight the major role of seagrass meadows in modification of seawater chemistry. Though the seagrass meadows of Palk Bay are increasingly subject to human impacts, with coupled regulatory and management efforts focused on improved water quality and habitat conservation, these key coastal ecosystems will continue to be valuable for climate change mitigation, considering their vital role in C dynamics and interactions with the overlying water column.