Changes in particulate and dissolved organic matter in nutrient-enriched enclosures from an area influenced by mucilage: the northern Adriatic Sea

Seasonal accumulation of carbon (C)-rich dissolved organic matter (DOM) recurs in many marine environments (Bjørnsen et al., 1989; Carlson et al., 1994) including some in the Mediterranean and Adriatic Sea (Copin-Montegut and Avril, 1993; Pettine et al., 1999; Tepić et al., 2000). In some cases decoupling of production and utilization with a net accumulation of DOM may be sustained on time scales of months to years (Church et al., 2002), and an ecosystem change of nutrient conditions [a shift from primarily nitrogen (N)-limited environment to phosphorus (P)-limitation] was hypothesized as a driving force (Karl et al., 1998). Recent analysis of the seasonal cycles of particulate and dissolved organic C and N in coastal waters revealed accumulations of C-rich DOM during productive periods, with C:N ratios well above the Redfield value of 6.6 (Williams, 1995). Reasons for this build-up of C-rich DOM are not clear but it has been hypothesized that lack of mineral N and/or P nutrients in the stratified water column prevents bacteria from taking up stored OM efficiently (Obernosterer and Herndl, 1995; Zweifel et al., 1995). Moreover, OM-degrading bacteria preferentially mineralize P with respect to C, leading to C-enriched OM (Smith et al., 1995). This C-rich OM is exported from the euphotic layer (Carlson et al., 1994) and/or degrades later in autumn. Thingstad and Rassoulzadegan (Thingstad and Rassoulzadegan, 1995) suggested that restricted bacterial degradation of DOM might result from a combination of nutrient limitation and predatory (viral) control. Seasonal accumulation of C-rich OM is thought to be the precursor of visible mucilage accumulations in the Adriatic Sea. Chemical analyses of mucilage material showed that C:N ratios of mucilage were 9.5–18.6 (Müller-Niklas et al., 1994) and indicated