Copepod carcasses in a tropical estuary during different hydrographical settings

Copepod carcasses in the western Bay of Bengal and associated ecology

Although a portion of all copepods in the natural environment are physiologically inactive or dead, there is little data on this aspect in Indian seas. Using the classical neutral red live staining technique, the dead percentage of copepods in the western Bay of Bengal (BoB) is determined for the first time in this study, which takes into account hydrography and zooplankton samples collected on April 2015 (Spring Intermonsoon [SIM]) from 6 inshore and 8 offshore locations in the western BoB. The offshore parts of the research area were infested with a persistent and extensive warm-core eddy/gyre during the sampling time, as demonstrated by satellite data of sea surface temperature, mean sea level anomaly, and surface currents. As a result, the mixed layer depth in the offshore zone was higher, whereas nutrients, chlorophyll a, and turbidity were significantly lower than inshore. Copepods dominated the zooplankton community (> 80%), with abundances ranging from 112 to 2580 No m^-3 in the study area. Calanoids (particularly Paracalanidae) dominated nearshore waters (35-65%), whereas cyclopoids (notably Corycaeidae and Oncaeidae) dominated offshore (49-71%). There were 48 species of copepods identified in total, and all of them had dead individuals (carcasses). Offshore waters had a larger percentage of copepod carcasses (27-39%) than inshore seas (18-28%). The larger percentage of carcasses is possibly related to a higher top-down feeding pressure on cyclopoids in offshore waters due to extreme oligotrophy and the dominance of the microbial food web.

Using osmotic shock to control invasive aquatic species

Coastal estuaries are especially vulnerable to the arrival and establishment of invasive aquatic species (IAS) as they are often the receiving locations of ship-based introductions. Rapid response tools, such as mechanical or chemical treatments to capture, remove, and contain of IAS, are needed to prevent subsequent spread into adjacent marine and freshwater systems. Abrupt salinity change, created when infrastructure in estuaries situated at the proximity of river mouths is operated, offers a novel, low-cost and environmentally friendly method for potentially controlling IAS. We investigated the use of osmotic shock to control the invasive brackish water clam Rangia cuneata that is quickly spreading through Europe. Clams were exposed in the laboratory to eight salinity concentrations ranging from 0.5 to 32.0‰ and monitored for 60 days. Saline shock, but not freshwater shock, could control R. cuneata. Salinities >26‰ killed 95% R. cuneata in two weeks. All specimens exposed to full strength seawater (32‰) were killed within 30 days. At lower salinities, clams collected from the most freshwater locality (1.2‰) showed lower mortality than clams from the most saline site (3.1‰). Furthermore, even modest increases in salinity during spawning periods of R. cuneata may prevent recruitment. Given the vulnerability of coastal estuaries to introduction of IAS, saline flushing presents a novel and effective management tool for many species.