Unusual winter zooplankton bloom in the open southern Adriatic Sea

Abstract

The paper reports an unusual response of the microzooplankton community to oceanographic conditions observed during the winter of 2015 in the open southern Adriatic. Record-breaking nauplii abundance of 13,734 ind. m -3 was sampled for the open southern Adriatic by 50-µm net sampling. This could be explained by (i) warmer-than-usual surface and intermediate ocean temperatures, (ii) higher precipitation that freshened and widened the surface layer, pushing saline Levantine Intermediate Water below a 400-m depth, and (iii) strong wind episodes that transported nutrients from the coastal zone to the open ocean and induced limited vertical mixing. Neritic tintinnids and viable photoautotrophs well below the photic zone support the last possibility. Average seasonal maximum of zooplankton abundance was shifted from spring and early summer to the late winter months. Our results document large and fast variations in production conditions rarely found to occur in oligotrophic waters such as the southern Adriatic Sea.

Tintinnid Ciliate Communities in Pre- and Post-Winter Conditions in the Southern Adriatic Sea (NE Mediterranean)

“…The community was characterized by the presence of surface and subsurface oceanic tintinnid species in April in the deep layers, which indicated that they can survive for more than a month in the deep, since vertical mixing at the open sea usually ceases by the end of February and beginning of March [28,33]. The greater part of the mesopelagic population was made up of subsurface oceanic species Undella claparedei and Codonella amphorella, especially in the layer from 100 to 300 m. This is in accordance with the vertical distribution of tintinnids in the open southern Adriatic during the winter convection events of 2008 [43] and 2015 [34]. Our data are comparable with those that were recorded in April 1986 in the open SA when some similar hydrographic/hydrological and biological characteristics were recorded: strong LIW inflow, downward transport, and phytoplankton bloom [12,50].…”

“…The high diversity of species and low abundances at the central stations of the SA, as well as high abundance at the perimeter stations towards the shoreline observed in December 2015, are a typical feature of the Adriatic Sea for winter and early spring that is already documented in many studies while using the same sampling methods [11][12][13][14]31,34]. The observed abundance distribution was positively correlated with the temperature and Chl-a.…”

“…The NIG is in a negative feedback relationship with the Adriatic Sea and the mechanism by which it changes its direction on a decadal scale is called the BiOS mechanism [27]. By modifying the properties of Adriatic water by the advection of different water masses through the Strait of Otranto, the BiOS mechanism can influence the intensity of phytoplankton blooms and production in the Adriatic [21,[28][29][30], as well as the ingression of new species [28,[31][32][33][34][35][36].…”

Mesozooplankton spatial distribution and community structure in the South Adriatic Sea during two winters (2015, 2016)

“…( Acartiura ) clausi , T. stylifera ) were recorded in the deepest layers in 2015, when Lučić et al. () noted neritic tintinnids below 500 m depth.…”

“…Chl a concentrations were several times higher in 2016, but owing to strong wind episodes and deep‐water convection in the winter of 2015, nutrients were advected from the coastal area to the open sea and vertically displaced. These unusual 2015 wintertime conditions were reflected in highly increased values of microzooplankton (nauplii and copepodites) down to the 300 m depth (same cruise – Feb–Mar 2015; Lučić et al., ).…”