Methods

Study area

The Gulf and the Bay of Panama are in the Eastern Tropical Pacific, on the southeastern coast of the Republic of Panama, comprising the geographic latitudes between 08 ° 38'N 079 ° 02'W and 08 ° 47'N 079 ° 27'W.  The Gulf of Panama is under the influence of the Intertropical Convergence Zone (ITCZ), where the trade winds from the northeast and southeast converge ( Xie et al., 2005; Amador et al., 2006). The presence of these winds are more intense during the dry season (January to April), giving rise to the oceanographic phenomenon known as upwelling (Forsbergh, 1969; Kwiecinski and Chial, 2016). El Niño and La Niña weather events tend to be closely related to upwelling intensity in the Gulf of Panama, resulting in very weak outcrops during El Niño and strong outcrops during La Niña (D’Croz et al., 2003).

Samples were taken at three sites named: Site 1: Las Perlas (8° 38.743' N; 79° 2.887' W) maximum depth of 20 - 25 meters, Site 2: Bay of Panama (8° 44.138' N; 79° 15.955' W) maximum depth of 45 – 50 meters and Site 3: Taboguilla (8° 47.078' N; 79° 27.968' W) maximum depth of 20 – 30 meters (Figure 1).

Environmental Sampling

Plankton tows and environmental sampling were conducted once a week between 7:30 AM and 2:30 PM, from April 2019 to March 2020. Seawater temperature, pH, salinity, and dissolved oxygen data sets were provided by the Physical Monitoring Program of the Smithsonian Tropical Research Institute . The water quality parameters were taken at each site with a YSI EXO2 Multiparameter Sonde (Figure 2) at the same time as the plankton were collected. The sonde logged data every second and was lowered rapidly with 3-minute stops every 5 meters, until it reached the bottom. 

Plankton sampling

We did three vertical hauls per site with a 250 μm mesh net attached to a 0.3-m-ring frame (SEA-GEAR model 9000) (Figure 3). Three diving weights of 1.37 Kg each were attached to the ring frame to certify a vertical sampling. The net was hauled vertically from 1 to 20 meters at the site 1 and site 3, and 40 meters at the site 2, above the sea bottom to the surface at a speed of 0.25 m/s. We recorded the site depth using a YSI EXO2 sonde attached to the boat and the sampling depth was measured using a rope with one-meter divisions. The volume of water filtered ranged from 1.06 m3 to 2.83 m3.

After the collection, the seawater volume was reduced using a 100 μm filter. Samples were placed in 45 ml centrifuge tubes and fixed with 4% formaldehyde to prevent organisms from shrinking. After 12 hours, the formaldehyde was replaced with 95% ethanol to preserve the sample. The zooplankton samples were examined in the laboratory using a stereomicroscope and sorted by genera or the lowest possible taxonomic level using morphological characteristics of the shell and eyes, following the websites The Tree of Life Web Project  World Register of Marine Species and the zooplankton guide from the Scripps Institution of Oceanography .