Export of particulate organic carbon and nitrogen to the northern Indian Ocean by the monsoonal rivers of India

This study analyses particulate organic carbon (POC) and particulate nitrogen (PN) export from Indian monsoonal rivers to the north Indian Ocean. Indian monsoonal rivers export approximately 1.2 Tg yr-1 (1Tg = 1012 g) of POC and 0.14 Tg yr-1 of PN, with about two-thirds entering the Bay of Bengal (0.8 and 0.1 Tg yr-1, respectively) and the remaining reaches to the Arabian Sea (0.4 and 0.04 Tg yr-1, respectively). Remarkably, just four rivers from northwest India's black soil-dominated regions contribute about half of the total POC and PN exports (0.64 and 0.06 Tg yr-1, respectively). This is due to substantial erosion in these catchments, resulting in suspended matter concentrations averaging 596 ± 252 mg L-1, significantly higher than catchments dominated by red sandy, red loamy and alluvial soils (54 ± 56 mg l-1). In contrast, rivers originating from catchments with heavy precipitation, a tropical wet climate, red loamy soils (with peaty and marshy characteristics), rich tropical wet evergreen and moist deciduous forests, and higher soil organic carbon content yield more POC and PN (1704 ± 383 kgC km-2 yr-1 and 261 ± 56 kgN km-2 yr-1, respectively) than the other rivers of India (951 ± 508 kgC km-2 yr-1 and 120 ± 57 kgN km-2 yr-1, respectively). These findings stress that the export flux and yield of POC and PN from the Indian monsoonal rivers are primarily influenced by the interplay of hydrological, lithological, environmental, and climatic conditions within the catchment, rather than river size. Moreover, this study highlights the significant impact of incorporating POC data from medium-sized rivers worldwide, as it reveals that yield is independent of river size. This calls for a re-evaluation of global POC export fluxes, taking into account hydrological, lithological, environmental, and climatic factors.

COVID-19 restrictions and their influences on ambient air, surface water and plastic waste in a coastal megacity, Chennai, India

Anthropogenic activities experienced a pause due to the nationwide lockdown, imposed to contain the rapid spread of COVID-19 in the third week of March 2020. The impacts of suspension of industrial activities, vehicular transport and other businesses for three months (25 March-30 June) on the environmental settings of Chennai, a coastal megacity was assessed. A significant reduction in the key urban air pollutants [PM_2.5 (66.5%), PM₁₀ (39.5%), NO₂ (94.1%), CO (29%), O₃ (45.3%)] was recorded as an immediate consequence of the reduced anthropogenic activities. Comparison of water quality of an urban river Adyar, between pre-lockdown and lockdown, showed a substantial drop in the dissolved inorganic N (47%) and suspended particulate matter (41%) during the latter period. During the pandemic, biomedical wastes in India showed an overall surge of 17%, which were predominantly plastic. FTIR-ATR analysis confirmed the polymers such as polypropylene (25.4%) and polyester (15.4%) in the personal protective equipment.

Spatial variations in dissolved inorganic nutrients in the groundwaters along the Indian coast and their export to adjacent coastal waters

Submarine Groundwater Discharge (SGD) is one of the main external nutrient sources to the coastal waters. The concentrations of nutrients in groundwaters are a few folds higher than that of adjacent coastal waters; therefore, SGD enhances nutrients levels in the coastal waters and influences coastal biota. In order to examine the spatial and seasonal variability in nutrient concentrations and exchange to the coastal waters, groundwater samples were collected at ~ 90 locations along the Indian coast during the wet and dry seasons. This study revealed that dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphates (DIP) and urea were found to be high during the dry than wet period. Higher concentrations of DIN and DIP were observed during both wet and dry periods in the groundwater along the east than the west coast of India. The State-wise mean amount of fertilizer used during Kharif (wet) and Rabi (dry) period in each Indian State showed significant correlation with mean concentrations of DIN and urea. The observed linear relationship of DIN with bacterial respiration and inverse relationship with DO saturation and ammonium in groundwater suggested that decomposition of organic matter and nitrification contributed to the DIN pool in the groundwater. The mean rate of SGD fluxes varied between 1.6 × 10⁴ m³/day and 1.75 × 10¹¹ m³/day in the Indian coastal region. The annual mean SGD flux of DIN and DIP was estimated to be 0.103 ± 0.02 and 0.021 ± 0.01 Tg (1 Tg = 10¹² g) to the western coastal Bay of Bengal (east coast of India) and 0.06 ± 0.03 and 0.015 ± 0.01 Tg/y to the eastern coastal Arabian Sea (west coast of India) respectively. The estimated SGD flux of DIN and DIP to the Indian coastal waters amounted to 0.163 ± 0.04 and 0.036 ± 0.02 Tg/y respectively, and it is almost close to that of nutrients discharged by rivers (0.22 ± 0.05 and 0.11 ± 0.03 Tg/y respectively). Among the external sources of nitrogen and phosphorus, such as river discharge, atmospheric deposition, the contribution by SGD is highly significant in the Bay of Bengal (30 and 17% respectively) than in the case of Arabian Sea (24 and 25% respectively).

Seagrass litter decomposition: an additional nutrient source to shallow coastal waters

Seagrass ecosystems are vital for its regulatory services yet, highly threatened by degradation due to human pressures. Decomposition of two tropical seagrass species (Cymodocea serrulata and Cymodocea rotundata) was studied and compared, to understand their potential in generating additional nutrients to coastal waters. Release of carbon, nitrogen and phosphorus during the decomposition process of seagrass wracks was estimated in bacteria-active (non-poisoned) and bacteria-inhibited (poisoned) conditions from shore-washed fresh seagrass, sampled from Palk Bay, India. Incubation experiments for 25 days indicated a near three times higher concentration of dissolved organic carbon (DOC) in bacteria-inhibited flasks compared to bacteria-active conditions for both species. The maximum leaching rates of DOC, TDN and TDP were found to be 294, 65.1 and 11.2 μM/g dry wt/day, respectively. Further, higher release of dissolved inorganic nitrogen (DIN) (> 1.3 times) was documented from the bacteria-active flask, highlighting the significance of microbial process in generating bio-available nutrients from decaying seagrass. Faster decomposition (0.014 ± 0.004 day^-1) in the initial stages (up to 8 days) compared to the later stages (0.005 ± 0.001 day^-1) indicated a rapid loss of biomass carbon during the initial leaching process and its relative importance in the decomposition pathway. The decomposition rate is best described by a single-stage exponential decay model with a half-life of 41 days. It is estimated that the total seagrass litter available along the Palk Bay coast is about ~ 0.3 Gg with high potential of additional nitrogen (0.9 ± 0.5 Mg) and phosphorus (0.3 ± 0.1 Mg) supply to the adjacent coastal waters.