Impact of decadal reversals of the north Ionian circulation on phytoplankton phenology

Abstract

Abstract. In the north Ionian, water circulation is characterized by a decadal alternation of cyclonic and anticyclonic regime driven by the mechanism called BiOS (bimodal oscillating system). The circulation regimes affect both vertical dynamics and the nutrient distribution. The north Ionian is then a good study area to investigate how changes in circulation can affect phytoplankton dynamics in oligotrophic regions. From in situ observations, for each circulation regime the averaged distribution of isopycnals is provided, and a depth difference of about 80 m is estimated for the nitracline between the cyclonic and anticyclonic regime. Based on phytoplankton phenology metrics extracted from annual time series of satellite ocean color data for the period 1998-2012, the cyclonic and anticyclonic regimes are compared. Results show that the average chlorophyll in March, the date of bloom onset and the date of maximum chlorophyll were affected by circulation patterns in the north Ionian. In the center of the north Ionian gyre, the bloom started in December and chlorophyll was low in March when circulation was anticyclonic, whereas during the cyclonic circulation regime, a late chlorophyll peak, likely resulting from different phytoplankton dynamics, was commonly observed in March. An additional analysis shows that the winter buoyancy losses, which govern the mixed layer depth (MLD), also contribute to explaining the interannual variability in bloom onset and intensity. Two trophic regimes were then identified in the north Ionian gyre (NIG) and they could be explained with the relative position of the MLD and nitracline.The first one is characterized by an early winter bloom onset and the absence of a chlorophyll peak in March. It was observed when circulation was anticyclonic or when winter MLD was relatively shallow. Dominant regenerated production all year and an absence of significant nutrient supplies to surface waters are proposed to explain this trophic regime. Conversely, the second trophic regime is marked by a bloom onset in late winter (i.e., February) and a chlorophyll peak in March. The chlorophyll increase was interpreted as a direct response to the nutrient enrichment of surface waters. This winter-spring bloom was observed when circulation was cyclonic and when winter mixing was relatively strong.

Drivers of the decadal variability of the North Ionian Gyre upper layer circulation during 1910–2010: a regional modelling study

“…Understanding the variability of the NIG circulation is of importance, since reversals between the two modes, affecting the spreading of the Atlantic Water (AW) eastward and salty Levantine waters westward, have a direct influence on the physical and biogeochemical properties of the adjacent regions (Civitarese et al 2010;Gačić et al 2011Gačić et al , 2013Mihanović et al 2015) and potential impacts on the broad marine ecosystem functioning (Lavigne et al 2018;Peharda et al 2018). Moreover, similar shifts of circulation regimes have also been observed in other regions of the world ocean (e.g.…”

Long distance particle transport to the central Ionian Sea

“…It is interesting to note that the southwestward jet sampled along ION-Tr is a persistent feature of Ionian Sea circulation, generally located at ca 20°E, 36°N and directed southward (see figure 2 Cyclonic circulation of the NIG implies a downwelling of nutricline at the northern border of the NIG, and an upwelling of nutricline in the central NIG (Civitarese et al 2010, Lavigne et al, 2018. Our observations of nutricline depth (isoline 1µM at 55m for SAV and ~95m for ST8) are consistent with the shallow nutriclines during cyclonic mode shown by Lavigne et al, (2018). Cyclonic mode also favors higher Chl-a in the center NIG, and a late winter bloom (Lavigne et al, 2018).…”

“…Circulation in the North Ionian, and thus the path of AW spreading, is variable and two alternate states have been proposed, the anticyclonic and cyclonic mode of the so called North Ionian Gyre (NIG, Gačić et al 2010). The direction of the NIG has a strong influence on the dispersal of water masses and properties in the Ionian basin, also impacting its productivity (Lavigne et al 2018).. Although the circulation in the Ionian basin has been documented according to seasons and NIG modes (Malanotte-Rizzoli et al 1997, Gačić et al 2011, Menna et al 2019, the fine scale pathways of AW crossing Ionian Sea have only been seldom sampled.…”

Copernicus Marine Service Ocean State Report, Issue 3

“…The geographical area of the spatial vorticity Figure 4.5.1. Summary of the water masses distribution in the Ionian Sea during the anticyclonic (left) and cyclonic (right) modes of the Northern Ionian Gyre (adapted from Lavigne et al 2018), superimposed with the yearly maps of absolute dynamic topography (colours) in 1996 and 1999, respectively. The acronyms in the figure are: Atlantic Water (AW); Levantine Surface Water (LSW); Levantine intermediate water (LIW).…”