Decadal changes of the Western Arabian sea ecosystem

Historical data from oceanographic expeditions and remotely sensed data on outgoing longwave radiation, temperature, wind speed and ocean color in the western Arabian Sea (1950–2010) were used to investigate decadal trends in the physical and biochemical properties of the upper 300 m. 72 % of the 29,043 vertical profiles retrieved originated from USA and UK expeditions. Increasing outgoing longwave radiation, surface air temperatures and sea surface temperature were identified on decadal timescales. These were well correlated with decreasing wind speeds associated with a reduced Siberian High atmospheric anomaly. Shoaling of the oxycline and nitracline was observed as well as acidification of the upper 300 m. These physical and chemical changes were accompanied by declining chlorophyll-a concentrations, vertical macrofaunal habitat compression, declining sardine landings and an increase of fish kill incidents along the Omani coast

Seafarer citizen scientist ocean transparency data as a resource for phytoplankton and climate research

The oceans' phytoplankton that underpin the marine food chain appear to be changing in abundance due to global climate change. Here, we compare the first four years of data from a citizen science ocean transparency study, conducted by seafarers using home-made Secchi Disks and a free Smartphone application called Secchi, with contemporaneous satellite ocean colour measurements. Our results show seafarers collect useful Secchi Disk measurements of ocean transparency that could help future assessments of climate-induced changes in the phytoplankton when used to extend historical Secchi Disk data

Long-term declining trend of zooplankton biomass in the Tropical Atlantic

We report evidences that the zooplankton biomass in the tropical Atlantic has declined with an almost 10-fold drop from the 1950s to 2000. The results of the multiple regression analysis showed that the decline in zooplankton biomass was positively related to the NAO-index and to phosphate concentration. We also found that the depth of the thermocline has decreased over the period of our investigation. Thus, the decline we report in zooplankton biomass may be related to the combined effect of two phenomena driven by global temperature increase: (1) the widening of the distributional range of tropical species due to the expansion of the ‘tropical belt’ and (2) a decrease in primary production resulting from the thinning of the thermocline. The decline of zooplankton biomass we report suggests that global warming of the ocean may be altering tropical food webs, and through them, it may also indirectly impact tropical oceans biogeochemical cycles

Seasonal dynamics of microzooplankton communities in the Sea of Oman (Arabian Sea)

Seasonal dynamics of microzooplankton and changes in environmental condition were analysed during a one-year field sampling campaign in the Sea of Oman at two different stations. Monsoon winds in this region cause distinct seasonality patterns with high primary productivity during the south-west monsoon in summer (June to October) and north-east monsoon periods in winter (November to March). Microzooplankton in the Sea of Oman showed several biomass peaks throughout the year. In general, higher biomass occurred during the south-west monsoon when compared to the north-east monsoon period with maxima of 190 µg C l−1at the inshore station Bandar Al-Khyran at 1m and 308 µg C l−1 at 10m water depth. At the offshore-station, peaks of 372 µg C l−1 (1m) and 256 µg C l−1 (20m) occurred during the south-west monsoon. A strong coupling between phytoplankton and microzooplankton was observed during monsoon periods but some microzooplankton peaked during inter-monsoon periods when chlorophyll concentration was low (Bandar Al-Khyran: 372 µg C l−1 at 1m and 196 µg C l−1, 10m; Offshore-station: 419 µg C l−1, 20 m). The initiation of phytoplankton blooms in the Sea of Oman was bottom-up controlled due to strong seasonal nutrient influx during south-west and north-east monsoon periods. Highest microzooplankton biomass occurred during monsoon periods with a dominance of Noctiluciphyceae and peaks of 7596 µg C l−1 at Bandar Al-Khyran (1m) and 5942 µg C l−1 (10m). Copepod nauplii, Amoebozoa and Larvacea contributed substantially to microzooplankton biomass throughout the year. Ciliophora contributed low proportion to the total microzooplankton biomass peaking both during monsoon and inter-monsoon periods. During the spring inter-monsoon, choreotrich ciliates (tintinnids) showed distinct peaks of 15.9 µg C l−1 at Bandar Al-Khyran (1m) and 17.7 µg C l−1 (10m) as well as 18.2 µg C l−1 at Offshore-station (20m). The interplay between bottom-up controlled primary production and top-down control mechanisms regulates the phenology patterns of specific microzooplankton groups in the Sea of Oman thus pointing at complex trophodynamic interactions at the lowermost foodweb level in this low-latitude ecosystem