INTRODUCTION

The Gulf of California is a highly productive area, characterized by having some of the highest surface nutrient concentrations of any ocean of the world (^{Santamaría-del-Ángel et al.,1994}). Additionally, it experiences great changes in productivity both seasonally and spatially. During winter months, high biological productivity occurs, while in the summer months, there is very low productivity (^{Santamaría-del-Ángel et al., 1994}). These peaks in productivity are highly influenced by a number of hydrodynamic changes, caused in part by changes in local wind speed and its direction, and solar radiation, which is representative of a semi-arid dessert region with elevated rates of evaporation of its surface waters (^{Obeso-Nieblas et al., 2007}; ^{Coria-Monter et al., 2017}). Furthermore, the Gulf of California is considered one of the most diverse seas of the world, inhabiting endemic and endangered or protected species, such as the whale shark (Rhincodon typus Smith, 1982).

In the Gulf of California, whale shark aggregations are known to occur in both Bahía de Los Ángeles and Bahía de la Paz (^{Eckert & Stewart, 2001}; ^{Ketchum et al., 2013}; ^{Whitehead et al., 2018}, ²⁰¹⁹) and seem to be related to high zooplankton abundance (^{Hacohen-Domené et al., 2006}; ^{Nelson & Eckert, 2007}). Initial work on prey preferences for the species have been investigated for Bahia de Los Ángeles (^{Nelson & Eckert, 2007}; ^{Lavaniegos et al., 2012}; ^{Hernández-Nava & Álvarez-Borrego, 2013}) and Bahía de La Paz (^{Clark & Nelson, 1997}; ^{Hacohen-Domené et al., 2006}; ^{Ketchum et al., 2013}). Known prey items on which R. typus have been observed to forage on in the Gulf of California, include a number of copepod species, mainly Acartia clausi, Acartia spp. and members the Euphausiidae family, such as Nictiphanex simplex (^{Clark & Nelson, 1997}; ^{Hacohen-Domené et al., 2006}; ^{Hernández-Nava & Álvarez-Borrego, 2013}; ^{Ketchum et al., 2013}).

Bahía de La Paz is the largest coastal water body in the Gulf of California with an approximate area of 2,635 km² and boasting both shallow and deep-water basins (^{Cruz-Orozco et al.,1996}; ^{Sánchez-Velasco et al., 2006}). Connectivity of the bay and its circulation is driven primarily by water exchange with the Gulf of California, which occurs by two passages, a northeasterly channel named Boca Grande and towards the southern part of the bay connectivity through the San Lorenzo Channel (^{Hernández-Trujillo et al.,1987}; ^{Monreal-Gómez et al., 2001}).

^{Ketchum et al. (2013)}, examined the foraging ecology of whale sharks in Bahía de La Paz and observed that plankton biomass varies seasonally, with the lowest in May and June and peaks of biomass occurring in November and February and all samples comprised of a mix of holoplankton and meroplankton taxonomic groups (^{Ketchum et al., 2013}). Earlier examinations of the composition of zooplankton present in Bahía de La Paz have shown that a number of species have a higher level of abundance during certain times of the year (^{Clark & Nelson, 1997}; ^{Hacohen-Domené et al., 2006}). ^{Clark and Nelson (1997)} witnessed a high density of copepods in the southern parts of the bay and reported that in this area composition seems to comprise mainly of Acartia clausi and Acartia sp. during the fall (^{Clark & Nelson, 1997}; ^{Hacohen-Domené et al., 2006}).

The area to the southern part of Bahía de La Paz, BCS, Mexico, encompasses a protruding sandbar attached at one end to the mainland and stretching out approximately 12 km parallel to the mouth of the bay, known as the El Mogote sandbar. The natural formation of this sandbar has created almost two separated bodies of water: the main bay itself and the inlet or lagoon known as the Ensenada de la Paz (^{León-de la Luz et al., 2006}). El Mogote covers an approximately 16 km² of surface vegetation comprised mainly of a sandy coastal environment with periodic patches of mangroves on its inlet side (^{León-de la Luz et al., 2006}). This region of Bahía de La Paz is thought to be influenced by its orientation to the mouth of the bay and changing environmental factors which occur throughout the year, such as seasonal changes in wind direction and magnitude and the circulation a central mesoscale cyclonic eddy, which is suspected to influence the horizontal distribution of all trophic groups of zooplankton in the bay (^{Durán-Campos et al., 2015}). This highly productive coastal zone experiences both primary and secondary productivity of planktonic species and is a known as the principle area for seasonally aggregating whale sharks (^{Ketchum et al., 2013}; ^{Ramírez-Macías et al., 2012}; ^{Whitehead et al., 2019}). Our study aimed to generate the first sampling annual cycle that covers all fours climatic seasons, to investigate changes in the overall abundance and composition of zooplankton members in the coastal waters off the El Mogote Sandbar, a foraging ground for juvenile whale sharks.

MATERIALS AND METHODS

Plankton trawls. A total of 96 zooplankton tows were routinely collected every two consecutive weeks from June 2016 through to May 2017 completing a full 12-month cycle of sampling in the coastal waters of the El Mogote sandbar. Plankton trawls were conducted at four predetermined sample stations, registered by GPS coordinates and encompassing the entire length of the el Mogote sandbar (Fig.1). All tows were performed in the surface waters using a 505 μm mesh plankton net with a 60 cm diameter mouth with a flowmeter and towed for five minutes behind the boat in a circular motion at approximately 1-1.5 knots h⁻¹ for the capture of macro zooplankton. Upon the net being removed from the water, records of the flowmeter rotations were reported and collected prey items were washed down into the collecting container at the end, fixed with a 4 % formaldehyde solution buffered with borax, labeled for identification and stored in plastic screw top containers. The total volume of wet zooplankton was determined by using the displacement method (^{Beers, 1976}) and the wet biomass was standardized using the standard biomass formula and the records collected from the flowmeter instrument to 100 m³ (^{Smith & Richards, 1979}). Total biomass was divided into four climatic seasons: 1) summer (June, July and August), 2) autumn (September, October, November), 3) winter (December, January, February) and 4) spring (March, April, May).

Figure 1 Study area showing sample stations (black circles) along the El Mogote sandbar in Bahía de la Paz, BCS, Mexico. 

Counting of prey items. Enumeration of organisms was done by extracting a subsample using a Stempel pipette, then calculating the total faunal count for the whole sample. Zooplankton was grouped into eight taxonomic groups: Cladocera, Copepoda, Chaetognatha, Euphausiidae, Hydrozoa, Decapoda, Fish larvae and “Other”. After zooplankton was assembled into groups, individuals were counted and standardized to obtain the total number of individuals (No. 100 m^-3) for each sample and ultimately an overall total number of individuals of planktonic groups for each climatic season.

Statistical Analysis. Following the collection and organization, data was analyzed in the statistical program R (^{R-Core Team, 2018}) using non-parametric statistics with a Kruskal-Wallis one-way analysis of variance test as a means to investigate any significant differences in the total biomass of zooplankton and the total number of individuals between climatic seasons and between taxonomic groups. Following this, a post-hoc Dunn test was applied to further explore were the variance exists within the total biomass and composition of zooplankton in collected samples.

RESULTS

Zooplankton Biomass. A seasonal shift in the overall biomass of zooplankton was observed during the study period with the lowest zooplankton biomass recorded in spring (176.28 ml 100 m^-3, 10.1%), while the highest values were observed for winter (703.04 ml 100 m^-3, 40.4%), followed by summer (493.92 ml 100 m^-3, 28.4%) and autumn (366.57 ml 100 m^-3, 21.1%) (Fig. 2a). A Kruskal-Wallis one-way analysis of variance test did show a statistically significant difference in total biomass between seasons (Kruskal-Wallis chi-squared = 29.993, p =0.000001385). A Post Hoc Dunn-Test revealed significant differences between almost all seasons, except for summer compared to autumn and winter (Table 1).

Figures 2a-b a) Seasonal averages of total zooplankton biomass in the waters off the El Mogote in Bahía de la Paz, BCS, Mexico; b) Seasonal average of the total number of individuals (sum of the eight taxonomic groups) in the waters off the El Mogote in Bahía de la Paz, BCS, Mexico. 

Zooplanktonic groups. Overall composition was largely comprised of three main taxonomic groups: Copepoda presented the highest concentrations of individuals (396,234 No. 100m^-3) accounting for 43.1% of the overall composition, followed by Chaetognatha (196,986 No. 100m^-3) 21.4% of the composition and Decapoda (116,513 No. 100m^-3) 12.6% of the composition, with the remaining five groups showing the lowest concentrations of individuals: Hydrozoa (50,475 No. 100m^-3) 5.4%, Cladocera (47,986 No. 100m^-3) 5.2%, Fish Larvae (40,906 No. 100m^-3) 4.4%, Other (36,625 No. 100m^-3) 3.9% and Euphausiida (33,967 No. 100m^-3) 3.7%. The total number of individuals (sum of the eight taxonomic groups) per season also showed significant differences (Kruskal-Wallis chi-squared = 29.78, p =0.000001533). The highest concentration of individuals was recorded in winter (292,429 No. 100 m^-3) accounting for 31.8% of the total amount of all individuals, followed by summer (243,489 No. 100 m^-3) 26.4% of individuals, autumn (220,779 No. 100 m^-3) 24% of individuals and spring (162,995 No. 100 m^-3) 17.7% of individuals (Fig. 2b). Results from the Post Hoc Dunn-Test explained that the difference in the number of individuals between seasons occurred in spring when compared to the other seasons (Table 2). In spring, the total number of individuals between taxonomic groups was significantly different (Kruskal-Wallis chi-squared = 84.615, p =1.571e^-15), with dominant taxonomic group being Copepoda (106,059 No. 100 m^-3, 65%), followed by Chaetognatha (25,282 No. 100 m^-3, 15.5%) and Fish Larvae (12,617 No. 100 m^-3, 7.7%) (Fig. 3).

In summer, the total number of individuals between taxonomic groups was also significantly different (Kruskal-Wallis chi-squared = 53.603, p =2.818e^-09). Copepoda (73,307 No. 100 m^-3, 30.1%) remained as the dominant taxonomic group followed by Decapoda (49,347 No. 100 m^-3, 20.2%), Chaetognatha (42,475 No. 100 m^-3, 17.4%) and Cladocera (37,940 No. 100 m^-3, 15.6%) (Fig. 3). In autumn, the total number of individuals between taxonomic groups was also significantly different (Kruskal-Wallis chi-squared = 72.406, p = 3.02e^-13) revealing Chaetognatha (75,420 No. 100 m^-3, 34.1%) to be the dominant taxonomic group followed by Copepoda (60,745 No. 100 m^-3, 27.5%) and Decapoda (27,605 No. 100 m^-3, 12.5%) (Fig. 3). Finally in winter, the total number of individuals between taxonomic groups was again significantly different (Kruskal-Wallis chi-squared = 84.615, p = 1.571e^-15) with the dominant taxonomic group being Copepoda (156,122 No. 100 m^-3, 53.3%) followed by Chaetognata (53,807 No. 100 m^-3, 18.4%) and Decapoda (30,335 No. 100 m^-3, 10.3%) (Fig. 3).

Figure 3 Number of individuals per taxonomic group per season in the waters off the El Mogote sandbar in Bahía de La Paz, BCS, Mexico. 

DISCUSSION

Understanding the productivity of a specific habitat, mainly at the base of the food web, may help to provide important information on the foraging behaviors of marine organisms that depend on it. A seasonal shift in the total biomass of zooplankton prey items was observed in our data, revealing that the largest accumulation of zooplankton biomass in the coastal waters of the El Mogote occurs in the winter months and lowest concentrations in the months around spring. An early study by ^{Jiménez-Pérez & Lara-Lara (1988)} examined zooplankton biomass and copepod community structure in the Gulf of California and in their observations showed that this region seems to present two unique community assemblages that may be related to different environmental factors caused by the shifting of climatic seasons that drive productivity in the entire region. Aggregations of whale sharks within the El Mogote area seem to be seasonal and have been documented to occur with a maximum peak during the winter months (^{Ramírez-Macías et al., 2012}; ^{Whitehead et al., 2019}). ^{Ketchum et al. (2013)} mentioned the segregation and foraging of whale shark in the southern Gulf of California, also reported seasonal correlation of juvenile whale sharks with zooplankton biomass in the coastal waters off the El Mogote. Whale shark aggregations and its correlation with changes in the abundance of available food sources has also been observed in other regions of the Gulf of California, such as Bahía de Los Angeles in the autumn months (^{Nelson & Eckert, 2007}) and in the Gulf of Mexico in the summer (^{Hoffmayer et al., 2007}; ^{Motta et al., 2010}). The size of the annual aggregation of juvenile whale sharks in Bahía de La Paz is suspected to be around 70 individuals (^{Whitehead et al., 2019}), but this number seems to vary among seasons and may be influenced by the presence of importance sources of macro zooplankton. An earlier study by ^{Ketchum (2003)}, proposed that the populations of whale sharks in Bahía de La Paz may be highly influenced by seasonal and inter-annual changes and anomalies of oceanographic patterns in the Gulf of California, such as El Niño years and annual hurricanes in the region. Seasonal changes in this region such as wind direction and magnitude may alter the natural circulation of the mesoscale cyclonic eddy in coastal waters, which is suspected to influence the entire horizontal distribution of macro groups of zooplankton as previously reported (^{Durán-Campos et al., 2015}).

Zooplankton composition in our study showed an overall dominance to the Copepoda. Absolute values of these individuals did show a significant difference throughout climatic seasons with a peak in the number of individuals recorded in the winter months and the peak of the seasonal aggregation of whale sharks (^{Whitehead et al., 2019}). Similar work on zooplankton and the foraging ecology of whale sharks in Bahía de La Paz have documented high concentrations of copepods during the autumn and winter months (^{Palomares-García, 1996}; ^{Hacohen-Domené et al., 2006}; ^{Ketchum et al., 2013}). Observations of juvenile whale sharks foraging on dense patches of zooplankton comprising of more than 90% Copepoda species in the southern regions of the bay near San Juan de la Costa (^{Clark & Nelson, 1997}), and concentrations of up to 80% Copepoda species in the waters in front of the El Mogote (^{Hacohen-Domené et al.,2006}) similar to our results. Likewise, in Bahía de Los Angeles a study focused on analyzing the composition of zooplankton, also reported a dominance of copepods, mainly in areas where whale sharks were sighted (^{Hernández-Nava & Álvarez-Borrego, 2013}). Daily or monthly changes in prey items within Bahía de La Paz and around the coastal waters of the El Mogote is highly influenced by its connectivity with water from the Gulf itself, which has been shown to be the main driving factor motivating water exchange (^{Monreal-Gómez et al., 2001}).

Historical reports from aggregations in the upper gulf (^{Hernández-Nava & Álvarez-Borrego, 2013}; ^{Nelson & Eckert, 2007}) and Bahía de La Paz (^{Hacohen-Domené et al., 2006}; ^{Ketchum et al.,2013}) together with our current report strongly support the perception that the presence of whale sharks is highly related to the number of Copepoda species available and that these planktonic organisms play an important role in the dietary preference of whale sharks in the entire Gulf of California. The presence of juvenile whale sharks coupled with the occurrence of an abundant food supply off the coastal waters of the El Mogote sandbar provides a much needed refuge for the development and fast growth of juvenile sharks (^{Wintner, 2000}; ^{Ketchum et al.,2013}; ^{Whitehead et al., 2019}). Now as whale shark related tourism is on the increase in this specific area, it is vital that continued work to monitor and provide good management of this ever growing industry alongside scientific research may help to understand the influences seasonal climatic changes have on the availability of food and its visiting sharks. In conclusion, this paper provides the first complete annual zooplankton sampling focused exclusively in the coastal water of the El Mogote area, and exposing that there is a clear fluctuation in zooplankton biomass and communities with respect to shifting environmental conditions and a solid baseline of information for this important aggregation area.