Cross-Shore Environmental Gradients in the Western Mediterranean Coast and Their Influence on Nearshore Phytoplankton Communities

Abstract

During summer, when oligotrophic conditions prevail offshore in the Mediterranean Sea, enhanced phytoplankton stripes are often observed in nearshore waters. In this study, we examine the cross-shore hydrographic variability and the associated microbial plankton communities in this zone. Detailed cross-shore underway sampling at 47 coastal sites spread along the Balearic and Catalan coasts revealed the widespread existence of narrow bands of warm and decreased salinity water beholding high phytoplankton biomass (up to 50-fold vs. offshore chlorophyll). Most intense physical and biological anomalies along these transects were generally constrained to the first hundred meters from the shoreline (i.e., a transition zone starting at ∼400 m). We use Principal Component Analysis (PCA) and k-means cluster analysis to categorize temperature, salinity and chlorophyll (T, S and Chl) in three main types of cross-shore trends. Prevalence of exponential-shaped Chl trends was observed particularly in areas with shoreward directed winds (B1-type). The other two trends (B2 and B3) presented variations off the coast produced by alongshore structures like river plumes, city outfalls and other features. Exponential-shaped cross-shore chlorophyll distribution (B1-type) accumulated 90% of the total transect Chl variation in the first 367 ± 190 m from the shoreline, whereas this distance was variable in the other profile types. Repeated daily sampling at one site with this transect typology revealed that wind forcing variations produced fast response on cross-shore T and S properties. Chl was less sensitive to changes at this time-scale. Phytoplankton communities exhibited site-dependent responses to the nearshore environment. Pico-and nanoplankton assemblages, typically dominating coastal assemblages during summer in the Mediterranean Sea, showed lower cross-shore variation. Conversely, larger response to nearshore conditions was observed in microplankton populations. These larger cells, represented by dinoflagellates, cryptophytes and diatoms, were able Basterretxea et al.Cross-Shore Gradients in Coastal Phytoplankton to actively exploit the nearshore conditions constituting an independent and distinct assemblage from that one prevailing offshore. Our results suggest that despite the importance of local-scale processes in determining biotic structure, some common patterns emerge providing clues on the main drivers of this nearshore niche.

Land Uses Simplified Index (LUSI): Determining Land Pressures and Their Link With Coastal Eutrophication

“…An example of the influence of other factors in the fitting of a pressure-impact relationship is that of the distance to the coast of the sampling stations when establishing this relationship between LUSI and the Biological Quality Element phytoplankton. The relationship between continental nutrients and chlorophylla is more direct at the coastline than in outermost waters, where nutrient availability decreases and other complex oceanographic mechanisms that affect phytoplankton growth come into play (Basterretxea et al, 2018). For Mediterranean Spanish coastal waters, the goodness of fit of pressure-impact relationships between LUSI and chlorophyll-a concentration was higher when chlorophyll-a was sampled at the coastline (62%) than at coastal outermost waters (51%) ( Table 6).…”

Assessment of microbial plankton diversity as an ecological indicator in the NW Mediterranean coast

“…These indices were developed to distinguish between natural and cultural eutrophication, which is key to management planning. The FLU index clearly distinguished samples from Alfacs Bay and L'Estartit, both influenced by rivers, as well as from L'Hospitalet de l'Infant which could be explained by the presence of nearby groundwater seeps (Fernández-Ruiz 2012, Basterretxea et al, 2018). Likewise, the FAN values were overall different among sites but varied slightly between Blanes and Barcelona despite these are a small and a large city, respectively.…”

“…For each descriptor, the status of the marine environment must be assessed using ecosystem criteria and indicators. There are currently multiple indicators being applied to the MSFD, some of them previously used under the European Water Framework Directive (WFD; 2000/60/EC), such as phytoplankton abundance and zoobenthos species composition, for eutrophication and biodiversity respectively (Borja et al, 2010;Camp et al, 2018). However, in the first case for example, the complexity of interactions between phytoplankton structure and physical, chemical and biological factors hinders the establishment of well-defined relationships between pressures and impacts, and therefore, effective management strategies.…”

“…Surface water samples were collected from six locations located along the Catalan and Balearic coastal areas ( Figure 1) that are representative of the NW Mediterranean coast in terms of geography, demography and socioeconomic activities. The choice of these coastal sites was based on previous characterization of the areas in the context of the Water Framework Directive (Table S1; Flo et al 2011Flo et al , 2017Flo et al , 2019 and on Basterretxea et al (2018). The six areas covered a variety of continental pressures and putatively receive variable nutrient loads and other pollutants from urban, industrial and agricultural activities (domestic waste, organic and inorganic nutrient enrichment among others).…”

“…First, four cross-shore transects were undertaken in Palma de Mallorca, L'Estartit, L'Hospitalet de l'Infant and Barcelona. Sampling was conducted in summer (June-July 2014 for the Catalan Coast and July 2015 for the Balearic Coast) when temperatures are warm (see Table 1) and there is a lack of tidal mixing (Basterretxea et al, 2018). Palma (39 • 32'N 2 • 43'E) is an intensive agricultural area in the island of Mallorca with reported nutrient rich groundwater seeps along the shoreline .…”