Changes in nutrient concentration and ratios during mucilage events in the period 1999–2002

Rudjer Bosković Institute, Center for Marine Research (CMR), G. Paliaga 5, 52210 Rovinj, Croatia.
Science of The Total Environment (Impact Factor: 4.1). 01/2006; 353(1-3):103-14. DOI: 10.1016/j.scitotenv.2005.09.010
Source: PubMed


Nutrient and chlorophyll a concentrations and salinity were measured, approximately monthly, from June 1999 to July 2002 at 20 stations along three transects in the northern Adriatic Sea, north of the line Susak Island-Senigallia, with the aim of confirming or rejecting the hypothesis that changes in nutrient ratios may have an important role in the mucilage phenomenon. The data analyses were focused on the two major water types identified in the region: lower salinity (32-37) and oxygenated surface waters (type 1) in which the mucilage phenomenon primarily developed, and high salinity water originating from other parts of the Adriatic (type 4). Marked variability of dissolved inorganic nitrogen (DIN) in type 1 waters was roughly correlated with extreme fluctuations of the Po River flow rate during the investigated period. In contrast, the orthophosphate (PO4) concentration was primarily controlled by phytoplankton assimilation. The nutrient discharges and DIN/PO4 ratios (median 120) in the freshened surface layer were much higher and more variable in the period before the mucilage event in 2001 than in 2000 (median 75), and particularly in 2002 (median 30), although intensity and duration of the 2001 event were the lowest. However, in that period of 2000 and 2002 significant transversal transport of freshened waters occurred, despite the unusually low Po flow rates. In summer, in conditions of low freshwater discharge and the prevailing of semi-enclosed circulation in the region, more efficient DIN assimilation by phytoplankton occurred, probably due to a faster recycling of PO4. However, in 2002 this process appeared to have already started in March. Changes of the orthosilicate (SiO4)/DIN ratio were mainly dependent on DIN concentrations. In the more saline waters (type 4) the nutrient concentrations, particularly DIN, were much lower and no significant relationships were noticed among the studied parameters. Nutrient concentration and ratio changes do not trigger mucilage events, although very probably they have an essential role in combination with several other physical (pulsing freshwater discharge, marked stratification, minimal water exchange) and biological (e.g., increased plankton excretion, limited bacterial degradation) factors.

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Available from: Tamara Djakovac, Jun 24, 2014
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    • "In shallow coastal areas such as the Gulf of Trieste, severe meteorological events may strongly modify the availability and the relative proportions of C, N and P pools in all compartments, as well as the oxygen conditions in the bottom layer (Gremare et al., 2003; Guadayol et al., 2009; Kaushal et al., 2010). Previous studies on organic matter cycling in the Adriatic (Obernosterer and Herndl, 1995; Lipizer et al., 1999; Danovaro et al., 2005) suggest that the uncoupling between C, N and P cycles may be among the possible triggering mechanisms leading to increased dissolved organic matter (DOM) production and to anomalous phenomena such as mucilage formation (Degobbis et al., 2005; Precali et al., 2005; Fonda Umani et al., 2007). The particulate matter plays a key role in transferring part of the C, N and P pools to the sea-bottom where the fraction which is not buried is remineralised or used by benthic organisms. "
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    ABSTRACT: To obtain more insight into the effects of severe forcing factors on a shallow coastal system, the elemental stoichiometry and the availability and partition of nitrogen, phosphorus and carbon in dissolved and particulate pools were assessed during events of particularly strong inputs of freshwater, high salinity anomalies, wind storms, algal blooms and elevated heterotrophic respiration processes. The research is based on data collected in the Long Term Ecological Research station of the Northern Adriatic Sea (Gulf of Trieste), from January 1999 to December 2010. During all considered events, stoichiometric ratios were higher than Redfield, due to an excess of carbon and nitrogen in relation to phosphorus. The particularly intense meteorological and biological events considered in this study altered the abundance, the relative availabilities of C, N and P and the stoichiometric ratios in different directions. Freshwater inputs and phytoplankton blooms caused a rise in the ratio between dissolved organic carbon and phosphorus, in N:P and C:P in the particulate compartment and, in the case of high freshwater only, in dissolved inorganic N:P, while the opposite was observed during events dominated by ingression of south-eastern waters and heterotrophic processes, when stoichiometric ratios decreased. Strong wind events, which are mainly due to north-easterly winds, did not seem to significantly modify the biogeochemical properties in the bottom layer.
    Full-text · Article · Dec 2012 · Estuarine Coastal and Shelf Science
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    • "Microscopic observations [7–11] indicate diatoms as their crucial producers although other phytoplankton species and bacteria are also present in macroaggregates. Concentrations of nutrients present during the marked retention of freshened waters and water column stratification in the northern Adriatic [12] seem to be less critical for mucous formation. Specific oceanographic conditions, including the formation of a gyre, higher seawater residence time, and development of a marked pycnocline during stable summer conditions with low turbulent shear [13], significantly contribute to the subsequent concentration and agglomeration of macromolecular organic matter, phytoplankton cells and other organics and minerals. "
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