Wind driven nutrient and subsurface chlorophyll-a enhancement in the Bay of La Paz, Gulf of California Erik Coria-Monter a, 1 , María Adela Monreal-G  omez b, * , David Alberto Salas de Le  on b , Elizabeth Dur  an-Campos a, 1 , Martín Merino-Ibarra b a Posgrado en Ciencias del Mar y Limnología, Universidad Nacional Aut onoma de M exico (UNAM), Ciudad de M exico, 04510, Mexico b Unidad Acad emica de Ecología y Biodiversidad Acu atica, Instituto de Ciencias del Mar y Limnología, Universidad Nacional Aut onoma de M exico (UNAM), Ciudad de M exico, 04510, Mexico article info Article history: Received 17 May 2016 Received in revised form 23 June 2017 Accepted 11 July 2017 Available online 13 July 2017 Keywords: Cyclonic circulation Nutrient enhancement Chlorophyll-a enhancement Bay of La Paz Gulf of California abstract Nutrient and chlorophyll-a distributions in the Bay of La Paz, Gulf of California, Mexico were analyzed during the late spring of 2004 to assess their relations to hydrography and circulation patterns. The results show the presence of both Gulf of California Water and Subtropical Subsurface Water. Water circulation was dominated by wind stress driven cyclonic circulation along f =H contours (f is planetary vorticity and H is depth), and upwelling resulting from the divergence shows a vertical velocity of ~0.4 m d 1 . Nutrient concentrations were higher in the center of the cyclonic pattern, where a rise in the nutricline contributed nutrients to the euphotic layer as a result of Ekman pumping. The vertical section showed the presence of a chlorophyll-a maximum at the thermocline shoaling to a depth of only 12 m. Along the surface, two peaks of chlorophyll-a were observed, one at Boca Grande and another off San Juan de la Costa, associated with upwelling and mixing derived from current interactions with abrupt topographies. The chlorophyll-a maximum increased from 0.8 mg m 3 in the external part of the cyclonic pattern to 2.0 mg m 3 in its center. The vertically integrated chlorophyll-a concentrations followed a similar pattern, rising from 10 to 20 mg m 2 and reaching their highest values in the center of the cyclonic circulation pattern. A schematic model was developed to describe processes that occur in late spring: the wind stress driven cyclonic structure promotes upward nutrient flux, which in turn drives an enhancement of chlorophyll-a. Upwelling was found to be the main mechanism of fertilization responsible for the enhancement of productivity levels by means of nutrient transport into the euphotic zone during spring. Other chlorophyll enhancement areas point to the occurrence of additional fertil- ization processes that may derive from interactions between cyclonic circulation patterns and the topography off of San Juan de la Costa, where phosphate mining occurs. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Wind stress driven cyclonic flows are considered to be impor- tant hydrodynamic processes that may affect the productivity, abundance, and distribution of plankton in the ocean at different space and time scales (Macías et al., 2008; Lee et al., 1992). Wind- induced currents are among the strongest in the upper ocean layer; the flow within the Ekman layer is divergent from cyclonic flows that induce upward vertical motion (upwelling) while the convergent flow drives vertical motion downward (downwelling) in an anticyclonic flow. Thus, cyclonic circulation induces an up- ward displacement of the nutricline, which penetrates the euphotic zone and which may enhance primary production and its expor- tation from this input of nutrients (Lee et al., 1992). Although numerous reports have been published on the biological effects of cyclonic and anticyclonic flows in open ocean regions, such studies on coastal areas of the southern Gulf of California and particularly on areas within bays are scarce. Nevertheless, within the Bay of La Paz, a mesoscale cyclonic structure has been observed on several occasions by various researchers and has been indistinctly referred to as a cyclonic eddy (Coria-Monter et al., 2014; Dur an-Campos et al., 2015), cyclonic circulation (Monreal-G omez et al., 2001), and cyclonic gyre (García-Mirafuentes, 2010; S anchez-Velasco et al., 2004, 2006). * Corresponding author. E-mail address: monreal@cmarl.unam.mx (M.A. Monreal-G omez). 1 Actual address: Becario Posdoctoral. Instituto de Ciencias del Mar y Limnología (UNAM). Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss http://dx.doi.org/10.1016/j.ecss.2017.07.010 0272-7714/© 2017 Elsevier Ltd. All rights reserved. Estuarine, Coastal and Shelf Science 196 (2017) 290e300