Horizontal and Vertical Distributions of Transparent Exopolymer Particles (TEP) in the NW Mediterranean Sea Are Linked to Chlorophyll a and O2 Variability

Abstract

Transparent Exopolymer Particles (TEP) are relevant in particle and carbon fluxes in the ocean, and have economic impact in the desalination industry affecting reverse osmosis membrane fouling. However, general models of their occurrence and dynamics are not yet possible because of the poorly known co-variations with other physical and biological variables. Here, we describe TEP distributions in the NW Mediterranean Sea during late spring 2012, along perpendicular and parallel transects to the Catalan coast. The stations in the parallel transect were sampled at the surface, while the stations in the perpendicular transect were sampled from the surface to the bathypelagic, including the bottom nepheloid layers. We also followed the short-term TEP dynamics along a 2-day cycle in offshore waters. TEP concentrations in the area ranged from 4.9 to 122.8 and averaged 31.4 ± 12.0 μg XG eq L−1. The distribution of TEP measured in transects parallel to the Catalan Coast correlated those of chlorophyll a (Chla) in May but not in June, when higher TEP-values with respect to Chla were observed. TEP horizontal variability in epipelagic waters from the coast to the open sea also correlated to that of Chla, O2 (that we interpret as a proxy of primary production) and bacterial production (BP). In contrast, the TEP vertical distributions in epipelagic waters were uncoupled from those of Chla, as TEP maxima were located above the deep chlorophyll maxima. The vertical distribution of TEP in the epipelagic zone was correlated with O2 and BP, suggesting combined phytoplankton (through primary production) and bacterial (through carbon reprocessing) TEP sources. However, no clear temporal patterns arose during the 2-day cycle. In meso- and bathypelagic waters, where phytoplanktonic sources are minor, TEP concentrations (10.1 ± 4.3 μg XG eq l−1) were half those in the epipelagic, but we observed relative TEP increments coinciding with the presence of nepheloid layers. These TEP increases were not paralleled by increases in particulate organic carbon, indicating that TEP are likely to act as aggregating agents of the mostly inorganic particles present in these bottom nepheloid layers.

Seagrass as major source of transparent exopolymer particles in the oligotrophic Mediterranean coast

“…Ortega-Retuerta et al (2010) already reported elevated TEP / Chl a ratios during early summer in the Mediterranean Sea, with values comparable to those we observe at the Faro Cap Ses Salines.…”

“…These observations suggest that P. oceanica meadows, the dominant ecosystem in Mediterranean coastal waters, are an important source of TEP precursors in the Mediterranean Sea (Ortega-Retuerta et al, 2010). Considering the average leaf production of P. oceanica of 876 g DW m −2 y −1 (Duarte and Chiscano, 1999), the estimated 37 000 km 2 covered by P. oceanica in the Mediterranean Sea (range 31 040 to 43 550 km 2 , Marbà et al, 2014) and the average TEP yield from leaf litter experimentally derived here (2344 µg C g DW −1 ), we calculated that P. oceanica releases about 76 Gg C as TEPs annually to the Mediterranean Sea.…”

“…Whereas TEPs are often assumed to be of phytoplankton origin, the relatively high levels of TEPs (i.e. high TEP / Chl a ratios) in oligotrophic Mediterranean waters suggest that DOC release by Posidonia oceanica meadows could be a source of TEPs, explaining the relative high TEP concentration reported for Mediterranean waters (Ortega-Retuerta et al, 2010). Although macroalgae have been identified as sources of TEPs, we are not yet aware of any study examining the role of seagrass as source of TEPs.…”

“…A total of 76 sampling events were completed at Faro Cap Ses Salines between 9 January 2012 and 23 March 2015, while 45 sampling events were completed at Es Caragol (from 9 August 2012 to 24 September 2014). Surface seawater samples of 250 mL from Faro Cap Ses Salines for chlorophyll a determination were filtered through Whatman GF/F filters and stored at −20 • C. Filters were extracted in 6 mL 90 % acetone for 24 h followed by fluorometric (Trilogy, Turner design) Chl a determination, calibrated with pure Chl a, after Parsons et al (1984). Sea-surface temperature was measured in situ using a data logger (HOBO).…”

“…P. oceanica meadows are highly productive (Duarte and Chiscano, 1999) and release high amounts of dissolved organic carbon (Barrón et al, 2014) as well as leaf litter (Cebrian and Duarte, 2001;Gacia et al, 2002). The large production of DOC and detritus by P. oceanica contrasts with the low planktonic primary production in the oligotrophic Mediterranean littoral zone , where TEPs are nevertheless present (Mari et al, 2001;Beauvais et al, 2003;Prieto et al, 2006;Ortega-Retuerta et al, 2010;Bar-Zeev et al, 2011) at levels higher than expected, as indicated by high TEP / Chl a and TEP / bacterial abundance ratios compared to other marine systems (Ortega-Retuerta et al, 2010. Whereas TEPs are often assumed to be of phytoplankton origin, the relatively high levels of TEPs (i.e.…”