title: Changes in nutrient concentrations and ratios during mucilage events in the period 1999-2002.
abstract: 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.
authors.affiliation: Rudjer Bosković Institute, Center for Marine Research (CMR), G. Paliaga 5, 52210 Rovinj, Croatia. degobbis@cim.irb.hr
authors.forename: Danilo
authors.initials: D
authors.lastname: Degobbis
authors.forename: Robert
authors.initials: R
authors.lastname: Precali
authors.forename: Carla R
authors.initials: CR
authors.lastname: Ferrari
authors.forename: Tamara
authors.initials: T
authors.lastname: Djakovac
authors.forename: Attilio
authors.initials: A
authors.lastname: Rinaldi
authors.forename: Ingrid
authors.initials: I
authors.lastname: Ivancić
authors.forename: Manuela
authors.initials: M
authors.lastname: Gismondi
authors.forename: Nenad
authors.initials: N
authors.lastname: Smodlaka
mesh_terms: D002734:Chlorophyll / Q000032:analysis; D000077194:Chlorophyll A; D056890:Eukaryota / Q000254:growth & development*; D008522:Mediterranean Sea; D009584:Nitrogen / Q000032:analysis; D009791:Oceanography / Q000706:statistics & numerical data*; D010100:Oxygen / Q000032:analysis; D010710:Phosphates / Q000032:analysis; D010839:Phytoplankton / Q000254:growth & development*; D011157:Population Dynamics; D012623:Seawater / Q000032:analysis*; D017640:Silicates / Q000032:analysis; D012965:Sodium Chloride / Q000032:analysis; D014872:Water Movements
chemical_list: D010710:Phosphates; D017640:Silicates; D002734:Chlorophyll; D012965:Sodium Chloride; D009584:Nitrogen; D010100:Oxygen; D000077194:Chlorophyll A