High-Chlorophyll-Area Assessment Based on Remote Sensing Observations: The Case Study of Cape Trafalgar

Abstract

Cape Trafalgar has been highlighted as a hotspot of high chlorophyll concentrations, as well as a source of biomass for the Alborán Sea. It is located in an unique geographical framework between the Gulf of Cádiz (GoC), which is dominated by long-term seasonal variability, and the Strait of Gibraltar, which is mainly governed by short-term tidal variability. Furthermore, here bathymetry plays an important role in the upwelling of nutrient-rich waters. In order to study the spatial and temporal variability of chlorophyll-a in this region, 10 years of ocean colour observations using the MEdium Resolution Imaging Spectrometer (MERIS) were analysed through different approaches. An empirical orthogonal function decomposition distinguished two coastal zones with opposing phases that were analysed by wavelet methods in order to identify their temporal variability. In addition, to better understand the physical-biological interaction in these zones, the co-variation between chlorophyll-a and different environmental variables (wind, river discharge, and tidal current) was analysed. Zone 1, located on the GoC continental shelf, was characterised by a seasonal variability weakened by the influence of other environmental variables. Meanwhile, Zone 2, which represented the dynamics in Cape Trafalgar but did not show any clear pattern of variability, was strongly correlated with tidal current whose variability was probably determined by other drivers.

Patterns and drivers of phytoplankton phenology off SW Iberia: A phenoregion based perspective

“…Over this turbid Case-II water masses, satellite-derived Chl-a retrievals could represent an overestimate, especially during high discharge periods (IOCCG, 2000;Caballero et al, 2014;Picado et al, 2014). However a direct relationship between river discharge and Chl-a was previously reported for this area (coastal northeastern GoC), using both OCRS (Navarro and Ruiz, 2006;Caballero et al, 2014;Krug et al, 2017b;Sala et al, 2018) and in situ collected data (Prieto et al, 2009;Huertas et al, 2006).…”

“…CSV is the northwest limit of the Gulf of Cadiz (GoC), a basin that connects the Mediterranean Sea and the Atlantic Ocean. The main continental influences over SWIP include topographic irregularities, such as prominent capes as CSV, Cape Santa Maria (CSM) and Cape Trafalgar (CT), and the Strait of Gibraltar, which affect coastal circulation dynamics (García-Lafuente et al, 2006;Sala et al, 2018). Freshwater inputs, particularly into the GoC area (e.g., Guadiana and Guadalquivir rivers discharge; Caballero et al, 2014; submarine groundwater discharges; Piló et al, 2018), are relevant local drivers of abiotic conditions and phytoplankton dynamics (Krug et al, 2017b).…”

“…The southwest area off the Iberian Peninsula (SWIP; NE Atlantic), located at a transition zone between temperate and subtropical waters, constitutes a highly heterogeneous domain, particularly vulnerable to climate change (Kovats et al, 2014). A wide diversity of processes, including local and large scale oceanic and atmospheric circulation patterns, topographic irregularities, coastal upwelling and continental freshwater outflows, impacts phytoplankton spatial and temporal dynamics (e.g., Navarro and Ruiz, 2006;García-Lafuente and Ruiz, 2007;Prieto et al, 2009;Navarro et al, 2012;Bruno et al, 2013;Goela et al, 2013;Caballero et al, 2014;Sala et al, 2018), promoting the occurrence of distinct regions where phytoplankton are driven, differently, by specific combinations of physical and climatic environmental drivers (see Krug et al, 2017b). Due to its geographical location (eastern boundary of the North Atlantic basin), SWIP and its complex coastal areas are often overlooked (Follows and Dutkiewicz, 2002;Vargas et al, 2009;Racault et al, 2012;Ferreira et al, 2014) or sparsely resolved (e.g., Siegel et al, 2002;Ueyama and Monger, 2005;Henson et al, 2009;Kahru et al, 2010;Martinez et al, 2011;Demarcq et al, 2012;D'Ortenzio et al, 2012;Sapiano et al, 2012;Land et al, 2014;Racault et al, 2014bRacault et al, , 2017González Taboada and Anadón, 2014;Cole et al, 2015;Cabré et al, 2016;Friedland et al, 2016Friedland et al, , 2018Zhang et al, 2017) in global or basin-scale phenological analysis.…”

Seasonality of Dissolved Organic Carbon Exchange Across the Strait of Gibraltar

“…In this sense, the seasonal cycle of primary production in the GC (Navarro & Ruiz, 2006) would be coupled to the fluctuations in DOC concentrations measured in the ASW in the same way as already described in BATS. Interestingly, a quasi‐permanent phytoplankton proliferation is found around the Trafalgar Cape (NW of the Strait), which has been attributed to local fertilization by the injection of nutrients from the MOW due to tidal effect, but which is particularly pronounced at the end of summer (Macías et al, 2007; Navarro & Ruiz, 2006; Sala et al, 2018). Macías et al (2007) also showed an eastward advection of phytoplankton toward the Alboran Sea by the Atlantic jet, which could well carry the DOC formed by the local bloom occurring at the end of summer, contributing to the DOC maximum detected over this period of the year.…”

Factors controlling pCO<sub>2</sub> variability in the eastern Gulf of Cádiz (SW Iberian Península)

“…In addition, the surface circulation in the Gulf of Cádiz is characterised by several different processes. These are: first the presence of an anticyclonic water flow towards the east over the shelf edge as far south as the Strait of Gibraltar, known as the Gulf of Cádiz Current (Sánchez and Relvas, 2003;Peliz et al, 2007); second, in the Trafalgar area an upwelling process occurs, produced by tidal interaction with the topography of the zone; and third, the mixing of surface layers induced by the wind (Vargas-Yáñez et al, 2002;Sánchez and Relvas, 2003;Peliz et al, 2009;Sala et al, 2018). In addition, the centre of the Gulf…”