Seasonal variability in copepod biomass in a cyclonic eddy in the Bay of La Paz, southern Gulf of California, Mexico

As one of the main groups composing marine zooplankton, copepods play an important role due to the position they occupy in the trophic web. Study of their biomass and relationship with the physical conditions of the water column are essential in order to evaluate the trophic structure and functions of any aquatic ecosystem. As a contribution to this topic, we assessed the copepod biomass inside a cyclonic eddy system during two different seasons in the Bay of La Paz in the southern Gulf of California, a region characterized by high biological productivity. Two oceanographic expeditions took place in the winter of 2006 and summer of 2009 on which a conductivity-temperature-depth (CTD) probe was used to determine the physical structure of the water column and oblique zooplankton hauls collected zooplankton samples. Satellite data were used to visualize chlorophyll-a distribution patterns. The results showed the presence of a well-defined mesoscale cyclonic eddy in both seasons, with high chlorophyll-a (CHLA) values at the edges of the eddy. Maximum values for copepod biomass were observed in winter and their distribution corresponded well with the circulation pattern and the CHLA values, forming a belt shape following the periphery of the eddy. The results presented herein highlight the impact of the mesoscale eddy on the planktonic ecosystem through its influence on hydrographic conditions in the water column. Other factors, such as ecological interactions, population dynamics, and feeding habits may play a role as well. Feeding behavior in particular is affected by high CHLA concentrations observed around the eddy which represent a source of food for these organisms.

Copepods are the most abundant multicellular organisms on Earth [1], [2] and in the 2 marine ecosystem where they are of prime importance due to the position they occupy 3 in the trophic web [3]. As mostly herbivorous organisms, they feed on phytoplankton 4 March 6, 2020 1/7 and therefore represent a link between lower and higher trophic levels. Several species of 5 copepods have high commercial value [4]. Additionally, trophic web dynamics 6 contribute to the removal of CO2 from the atmosphere, through sedimentation of 7 inorganic and organic carbon compounds included in fecal pellets, and to the 8 appropriate functioning of the biological or carbon pump [3]. 9 Typically, zooplankton biomass is indicative of secondary production and estimation 10 of this parameter is essential to evaluate trophic structure and function in any aquatic 11 ecosystem [5]. Changes in zooplankton biomass are closely related to several factors, 12 including variations in the salinity field [6], the temperature regime [7], and the 13 availability of food [8]. 14 Another source of variability in zooplankton biomass is the presence of 15 hydrodynamic processes, which modify the hydrographic structure of the water column 16 and exert a remarkable effect on productivity by introducing nutrients into the euphotic 17 zone, with consequent enhancement in phytoplankton biomass due to the increased 18 availability of food for the zooplankton community [9]. These hydrodynamic processes 19 are present throughout the water column at different scales, including internal waves, 20 fronts, and eddies [10]. 21 Mesoscale eddies (radii 10-100 km) are high energy hydrological structures of prime 22 importance in any marine ecosystem [11]. These structures, are recognized as cyclonic, 23 anticyclonic, and mode-water with a noticeable impact on the planktonic ecosystem [12]. 24 It has been demonstrated that the presence of cyclonic eddies modulate the structure 25 and biomass of the zooplankton community in different parts of the world, including the 26 Mediterranean Sea [13], the Sargasso Sea [14], the Madagascar Channel [15], the Pacific 27 Ocean off central-southern Chile [16], and the Hudson Bay in Canada [9].

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Although the impact of mesoscale eddies on zooplankton biomass has been relatively 29 well demonstrated worldwide, uncertainties remain about the role of these structures on 30 particular groups of zooplankton, in this case copepods. As a contribution to this topic, 31 this research aimed to assess the biomass of copepods in a mesoscale cyclonic eddy 32 system in the Bay of La Paz, Gulf of California. Hydrographic data and zooplankton 33 samples were collected during two oceanographic expeditions in two contrasting seasons, 34 summer 2009 and winter 2006. We hypothesized that the copepod biomass will vary 35 with respect to season and hydrodynamics of the mesoscale eddy system. It is our 36 intention that this study contributes to a better understanding of the influence of 37 mesoscale eddies on particular and pivotal zooplankton groups, such as copepods, in one 38 of the most productive marine ecosystems in the world.

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The Bay of La Paz is the largest basin in the Gulf of California. The region is highly 41 dynamic, with a wide seasonal and interannual variability due to atmosphere-ocean 42 interactions. In winter, the prevailing winds are predominantly northwesterly, with high 43 and persistent speeds exceeding 10 m s -1 . In summer, southeasterly winds blow at 44 approximately 5 m s -1 , with frequent calms [17].

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The study area is recognized for its high biodiversity which has been linked to the 46 hydrodynamics of the region, involving the presence of a quasi-permanent cyclonic eddy 47 which exerts a great impact on the planktonic ecosystem by supporting production at 48 high trophic levels. Indeed, this cyclonic eddy promotes a nutrient-rich Ekman pump 49 that fertilizes the euphotic layer [18] and induces a differential distribution in 50 phytoplankton between diatoms and dinoflagellates [19] and a differential aggregation of 51 zooplankton inside the eddy field [20], which then impacts the whole pelagic food web. 52 In this research, high-resolution hydrographic data and zooplankton samples were

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In order to establish the coupling between the circulation patterns obtained at the 121 time of our observations of CHLA concentrations and the copepods studied (calanoids, 122 cyclopoids, and all copepodite stages), the results of the biomass for each group were 123 superimposed (Fig. 2). The results showed a pattern of distribution with progressive 124 changes from the connection with the Gulf of California to the interior of the bay, as 125 well as from the periphery to the center of the cyclonic eddy.

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In winter, biomass concentrations were slightly higher than those observed in 127 summer, showing two regions of maximum biomass for the three target groups: one 128 located at the junction with the gulf, one in the Boca Grande region, and a third in the 129 western region close to the coast (> 5 m/100 m 3 ) (Fig. 2 a-c). A pattern of change was 130 observed within the eddy field with high values associated with the periphery of the 131 eddy and defining its circumference. During the summer, the highest values, located in 132 the western region close to the coast (> 5 mg/100 m 3 ), coincided with secondary high 133 values around the periphery of the eddy (Fig. 2 d-f).

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These results showed that the copepod groups were influenced by the cyclonic eddy 135 through their actions on the hydrographic conditions, and possibly as a result of several 136 additional processes such as ecological interactions, population dynamics, and feeding phytoplankton communities, and thus for the zooplankton grazers [24]. In particular, 144 copepods are one of the most successful organisms in the marine environment because 145 their torpedo shape enhances mobility which is propitious for locating food [2]. Along 146 this line of thought, the high copepod biomass values found in the three groups 147 analyzed near the connection with the Gulf of California can be explained by the 148 presence of a bathymetric sill where important processes take place (e.g., hydraulic 149 jumps) that fertilize the euphotic zone [25]. The highest biomass was observed in the 150 western portion of the bay close to the coast, and can be explained by the presence of a 151 phosphate mining industry that fertilizes the region, causing phytoplankton blooms that 152 increase the herbivorous zooplankton population. The progressive changes observed in 153 copepod biomass around the eddies could be induced by the advection generated by 154 convergent movements induced by the cyclonic structure.

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The low concentration of CHLA observed in the center of the eddy in both seasons 156 could be related to the predominance of certain heterotrophic phytoplankton groups 157 (e.g., dinoflagellates), while the high copepod abundance associated with high CHLA 158 values in the form of a belt-shaped area are related to the predominance of diatoms, as 159 previously reported by [19]. 160 Changes observed in phytoplankton biomass according to season could be associated 161 with normal periods of heating and cooling of surface layers in summer and winter, 162 which induce mixing in winter, thus increasing the concentration of nutrients leading to 163 high CHLA values [26] available for the zooplankton. Martínez-López et al. [27] also 164 reported this pattern of high biological productivity during winter in Alfonso Basin.