Seasonal Variations of the Zooplankton Community in the Western Gulf of Mexico: is there an Influence of the Warm Eddy Jumbo?

© 2024 Academia Sinica, Taiwan Open Access Seasonal Variations of the Zooplankton Community in the Western Gulf of Mexico: is there an Influence of the Warm Eddy Jumbo? Elia Lemus-Santana1,*, Laura Sanvicente-Añorve2, and Miguel Alatorre-Mendieta3 1Posgrado en Ciencias del Mar y Limnología. Universidad Nacional Autónoma de México. Av. Universidad 3000, Ciudad Universitaria Coyoacán, C.P. 04510, Mexico City, Mexico. *Correspondence: E-mail: [email protected] (Lemus-Santana) 2Laboratorio de Ecología de Sistemas Pelágicos Instituto de Ciencias del Mar y Limnología. Universidad Nacional Autónoma de México, Circuito Exterior S/N, Ciudad Universitaria, C.P. 04510, Mexico City, Mexico. E-mail: [email protected] (Sanvicente-Añorve) 3Laboratorio de Oceanografía Física, Instituto de Ciencias del Mar y Limnología. Universidad Nacional Autónoma de México, Circuito Exterior S/N, Ciudad Universitaria, C.P. 04510, Mexico City, Mexico. E-mail: [email protected] (Alatorre-Mendieta) Received 14 December 2023 / Accepted 27 November 2024 / Published 27 December 2024 Communicated by Ryuji Machida In the Gulf of Mexico, the Loop Current sporadically sheds warm anticyclonic eddies that travel into the gulf and whose influence on the zooplankton community of the western region is not known. This research examined the zooplankton community dynamics in the western Gulf of Mexico during three seasons: July 2010 (summer), January 2011 (winter), and October–November 2012 (fall), and the possible effect of the warm eddy called Jumbo, released from the Loop Current in the middle of 2012 and that approached the western side of the gulf at the end of the year. We hypothesized shifts in the composition and/or biomass of the zooplankton fauna collected during the fall period due to the transport of organisms from elsewhere or because of a rapid response of zooplankton to warmer environmental conditions. This could result in a greater similarity of the fall season to the summer rather than to the winter. Zooplankton samples were taken onboard the oceanographic vessel Justo Sierra and a total of 82 oceanographic stations were sampled with a Bongo net; at each sampling station, temperature and salinity were measured with a CTD profiler. Both environmental and zooplankton data were treated through a Principal Coordinate Analysis (PCO) to explore their relationship. Fourteen zooplankton groups were recognized in all three sampling periods, with seasonal variations in biomass. The PCO showed that July was characterized by high-temperature values (~27°C), low chlorophyll concentration (< 1 mg/m3), the dominance of copepods, chaetognaths, and luciferids, as well as high biomass values of crustacean larvae (decapods stomatopods), signaling this season as the reproductive period. January was characterized by higher chlorophyll concentration (1–1.3 mg/m3), lower temperatures (18–22°C), and a high biomass of amphipods, ostracods, and jellyfishes; October–November registered similar environmental conditions to July, but the PCO and the associated distance among centroids indicated that the zooplankton community structure was more similar to January. The occurrence of the 14 groups in all the seasons, reveals no shifts in the composition in the study area. Besides, the similarity of the fall to the winter in the zooplankton structure discarded the hypothesis. Our results suggest that the zooplankton community follows its natural seasonal dynamics and shows high resilience to eventual hydrographic phenomena, such as anticyclonic eddies. Key words: Anticyclonic eddies, Temperature trend, Reproduction, Zooplankton resilience, Sea surface temperature (SST) Citation: Lemus-Santana E, Sanvicente-Añorve L, Alatorre-Mendieta M. 2024. Seasonal variations of the zooplankton community in the western Gulf of Mexico: is there an influence of the warm eddy Jumbo? Zool Stud 63:57. doi:10.6620/ZS.2024.63-57. Zoological Studies 63:57 (2024) doi:10.6620/ZS.2024.63-57 1 © 2024 Academia Sinica, Taiwan BACKGROUND The Gulf of Mexico is a semi-enclosed sea, where circulation patterns result from complex interactions between bathymetric mechanisms, wind forcing, atmospheric conditions, water density, and the Loop Current (Oey et al. 2005; Sturges and Kenyon 2008). In the central zone of the gulf, circulation is determined by the Loop Current and the anticyclonic eddies that shed from it. When an anticyclonic eddy sheds, its trajectory starts toward the western or west northwestern gulf, where it meets the continental shelf between the northern end of Veracruz (Mexico) and the south of Texas, and where it dissipates (Vidal and Vidal 1997; Hamilton et al. 1999; Hamilton and Berger 2002; Muller-Karger et al. 2015). In their trajectory, eddies generate currents and redistribute warm and cold water on the ocean surface to ~1000 m depth; therefore, they affect the structure and distribution of the plankton (Biggs et al. 1997; Zavala-Hidalgo and FernándezEguiarte 2006). Anticyclonic eddies travel toward the interior of the gulf at speeds of ~6 km/d, with residence times of about 9 to 12 months and shedding intervals from 3 to 17 months (Vidal and Vidal 1997; Sturges and Leben 2000; Oey et al. 2005; Hamilton et al. 2019). In June 2012, a specific phenomenon occurred in the western central Gulf of Mexico: an anticyclonic eddy called ‘Jumbo’ was shed from the Loop Current. The eddy Jumbo was characterized by its large size and long duration. In August 2012, it reached a diameter of approximately 247 km, and lasted for about nine months. Once it collided with the continental slope, it dissipated in the northern Veracruz-southern Tamaulipas area, two Mexican states bordering the western gulf (Díaz-Maya 2018; WHG 2023). Besides this sporadic mesoscale hydrographical phenomenon, the river plumes generated in the area modify the physical, chemical, and biological conditions of the pelagic environment (Cruz-Ábrego et al. 1991; Biggs and Ressler 2001; Dagg and Breed 2003). All these mesoscale processes on the continental shelf and the open water of the western gulf drive shifts in the dynamics of the plankton communities. A vital function of zooplankton in the pelagic ecosystem is the transmission of energy from primary producers to higher trophic levels. Since the organisms of the zooplankton have short life cycles (days, weeks, months, or occasionally years) they respond rapidly to environmental changes (Verity and Smetacek 1996; Litchman et al. 2013). Therefore, understanding the shifts in the zooplankton community structure in response to environmental changes at different scales will allow us a better understanding of the marine pelagic ecosystem of the Gulf of Mexico. The zooplankton community on the western side of the Gulf of Mexico has been scarcely addressed, encompassing only a few zooplankton groups (LópezSalgado et al. 2000; Gutiérrez-Aguirre et al. 2015; Sanvicente-Añorve et al. 2021). To continue the zooplankton exploration in this region, three field surveys were conducted during July 2010 (summer), January 2011 (winter), and October–November 2012 (fall) to examine the seasonal variations of the whole zooplankton community and the possible influence of the eddy Jumbo, that approached the western gulf at the end of 2012, on the hydrological conditions and zooplankton community. We hypothesized shifts in the zooplankton structure of the fall period due to the transport of organisms from the eastern or central gulf or because of a rapid response of zooplankton to warmer water conditions. This could cause the fall period to be more similar to the summer than to the winter. MATERIALS AND METHODS Field and laboratory work The zooplankton samples were taken as part of the project “Environmental Framework of the Oceanographic Conditions in Mexico’s Northwestern Exclusive Economic Zone in the Gulf of Mexico” (MARZEE). Sampling was carried out in July 2010, January 2011, and October‒November 2012 (Fig. 1) onboard the oceanographic vessel Justo Sierra. In total, 82 stations were sampled using a Bongo net (333 and 505 μm mesh); in each net, a flowmeter was installed to estimate the volume of filter water. The samples were collected following oblique tows; sampling depth varied between 10 to 200 m and towing time between 4 to 25 min, both depending on the bottom depth. Samples were fixed with 4% formaldehyde and neutralized with sodium borate. In each oceanographic station, temperature and salinity were measured with an SBE 9Plus CTD profiler, and chlorophyll concentrations were measured with a Wet Labs FLRTD sensor adapted to the sonde. All measurements were read from the surface down to 1500 m depth, depending on the bottom depth of each station; however, they were graphically represented by taking the mean integrated value in the 50 m surface layer. In the laboratory, the samples obtained with the mesh size of 333 µm were processed with a Folsom plankton splitter; samples were divided from one to five times depending on the whole zooplankton concentration. From each fraction, the main groups of zooplankton were separated and identified according page 2 of 16Zoological Studies 63:57 (2024) © 2024 Academia Sinica, Taiwan to specialized literature (Gasca and Suárez 1996; Boltovskoy 1999a b; Johnson and Allen 2012; Castellani and Edwards 2017). Since the whole zooplankton community was taken into account, organisms were classified into taxonomic groups broader than the superfamily level. Afterward, the biomass of each zooplankton group from each sampling station was estimated by removing the interstitial water between Fig. 1. Study area and sampling sites during the three seasons in the western Gulf of Mexico. A: Cruise MARZEE 1, 31 sampling stations; B: Cruise MARZEE 2, 25 sampling stations; C: Cruise MARZEE 3, 26 sampling stations. page 3 of 16Zoological Studies 63:57 (2024) © 2024 Academia Sinica, Taiwan the organisms and transferring the zooplankton to a measuring cylinder with a known volume of water; the displaced volume was standardized to 100 m3 of water and represented the biomass of each zooplankton group in mL/100 m3 (Sell and Evans 1982; Postel et al. 2000). Anticyclonic eddies Two main eddies were released from the Loop Current during the sampling period, Icarus and Jumbo. According to their size, the Icarus was categorized by scientists as a ‘large’ eddy (175 km in diameter approximately), whereas the Jumbo, was considered a ‘huge’ one (247 km in diameter approximately). The Icarus detached from the Loop Current in November 2011 and its lifetime ended in February 2013; at the end of its life, it lost strength and became part of the circulation in the southwest of the Gulf (Hamilton et al. 2016; WHG 2023) (Fig. 2). The Jumbo was released from the Loop Current in June 2012 and dissipated in February 2013; this eddy split into two quasi-separate eddies. At the end of its life, the Jumbo integrated into the circulation pattern in the southwest of the Gulf (Díaz-Maya 2018; WHG 2023) (Fig. 2). Until September 2012, the eddies Icarus and Jumbo were well differentiated, but after that date, it appears that their trajectories joined and dissipated in the southwestern Gulf. Data analyses A principal coordinate analysis (PCO) was performed to evaluate the temporal variation in the zooplankton community. This analysis ordinates both environmental and biological similarity matrices allowing us to visualize the proximity between them (Anderson et al. 2008). This procedure was applied three times. The first one included only environmental parameters (temperature, salinity, and chlorophyll), using a Euclidean distance similarity algorithm with data previously transformed into Log (x+1) and normalized. The second analysis included only biological data (composition and biomass of zooplankton taxa); data was Log (x+1) transformed and the similarity matrix was constructed using the Bray-Curtis index. The third analysis included both matrices (environmental + biological) with the same data treatment mentioned above. In all the analyses, a Pearson correlation higher than 0.5 was employed. In all the cases, a similarity analysis (ANOSIM) was applied to determine if there were significant differences among seasons. Additionally, the distance among the centroids of the seasons was calculated from the PCOs ordination diagrams to estimate the degree of similarity between each pair of seasons. All analyses were performed using the PRIMER v.7.0.13 software (Clarke and Gorley 2015). The friction depth, or the Ekman pumping depth, was estimated in some transects to help the understanding of some hydrological characteristics. The friction depth (De), indicating the depth at which the wind-driven current becomes insignificant compared to velocity in surface waters, was estimated according to the following equation (Pond and Pickard 1983; Li et al. 2021). De = 4.3Ua √sin(|θ|) where, Ua = wind speed (m/s) θ = latitude (radians) Surface wind velocities were taken from the Windy Weather Service platform (Windy 2024) for Tamaulipas, and corresponded to 2.64, 2.81, and 3.87 m/s for July, October–November and January, respectively. Finally, the web resource Atlantic Oceanographic and Meteorological Laboratory of the NOAA (NOAA-AOML 2023) was consulted to complete the temperature data for the hydrological analyses. This web includes monthly and annual time series of the sea surface temperature in the Gulf of Mexico since 1985. All median annual temperatures from 1985–2022 were taken and graphed to visualize the temperature trend in the study area. RESULTS The zooplankton in their natural environment The temperature values in July and OctoberNovember were high, around 27°C (Fig. 3A, 3C); the highest values were recorded in July in the northern end of the study area and, two colder patches (26°C) were also observed (Fig. 3A). In January, the temperature showed a clear coastal-ocean gradient, with the lowest values (18.7°C) recorded in the coastal area and the highest (22.8°C) in the oceanic zone (Fig. 3B). Vertical temperature profiles of transects 2 and 5 (from north to south) in July showed an upwelling of colder subsurface water to the surface (Fig. 4) and, estimations of the friction depth in those transects were 17.4 and 17.9 m, respectively. As seen, the friction depth values nearly correspond to the height of the dome resulting from page 4 of 16Zoological Studies 63:57 (2024) © 2024 Academia Sinica, Taiwan the upwelling of subsurface waters (Fig. 4). This could explain the colder patches observed in July in the temperature horizontal planes (Fig. 3A). Considering a greater temporal scale, and treating the NOAA historical records, the temperature in the Gulf of Mexico had a mean increase of 0.48°C from 1985 to 2022 (i.e., an increase rate of 0.013°C per year) (Fig. 5). The salinity was less variable. In July it ranged from 36 to 36.5 psu, with the lowest values in the northern study area and off the Soto la Marina River discharge. January recorded the greatest gradient, with the lowest values (33 to 35 psu) on the continental shelf and the highest (36 psu) in the oceanic zone. Finally, October–November had small salinity variations (36.3 to 36.6 psu) (Fig. 6). The chlorophyll concentration was low in the Fig. 2. Trajectories of the eddies Icarus (black circle) and Jumbo (red circle) between 2011–2013 in the Gulf of Mexico. Images taken and modified from an animation from the Copernicus (2024) website. page 5 of 16Zoological Studies 63:57 (2024) © 2024 Academia Sinica, Taiwan three sampled seasons (< 1.3 mg/m3). In January, the highest values of chlorophyll concentration (1 to 1.3 mg/m3) were recorded near the mouth of the Soto la Marina River. During July and October–November, the chlorophyll values ranged from 0.09 to 1 mg/m3 (Fig. 7). The mean zooplankton biomass values varied among the seasons, with the highest records (14.94 mL/100 m3) in July and the lowest in October– Fig. 3. Mean integrated values of temperature (°C) in the upper 50 m layer in the western Gulf of Mexico. A: July 2010, summer; B: January 2011, winter; C: Oct/Nov 2012, fall. page 6 of 16 Zoological Studies 63:57 (2024) © 2024 Academia Sinica, Taiwan November (4.83 mL/100 m3) (Table 1). The zooplankton was classed into fourteen major groups broader than the superfamily level (amphipods, chaetognaths, copepods, decapod larvae in megalopa stage, fish larvae, jellyfishes, luciferids, mollusks, ostracods, polychaetes, salps, shrimp-like decapod larvae, siphonophores, stomatopod larvae). All the groups occurred in all the sampling seasons and most taxa were more abundant Fig. 4. Vertical temperature profiles of the transects 2 (A) and 5 (B) (from north to south) in July 2010, summer. The dotted lines indicate the friction depth. Fig. 5. 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