Mucilage Events Research Articles

Dynamics of the oceanographic properties during mucilage appearance in the Northern Adriatic Sea: analysis of the 1997 event in comparison to earlier events

The dynamics of the oceanographic properties in Northern Adriatic Sea during the 1997 mucilage event is reported. These observations, compared to a longer time series (1985–1998), indicate that processes on short temporal scales and abrupt changes of the physical and biogeochemical conditions in the basin have a great effect on mucilage events. The seawater warming during the years with the mucilage was delayed in May and June compared to those without aggregate. Since the same typical summer values were afterward reached in both cases, a more abrupt warming is supposed in the period preceding mucilage events. During 1997, the Po River regime had an abrupt change from very low flow rates in April–May (526–985 m 3 s −1) to a strong freshet in July (4870 m 3 s −1). This river pulse, coupled to the scarce intrusion of the high salinity Middle Adriatic Deep Water (MAdDW), determined the large retention of low salinity waters in the Northern Adriatic Sea from July to September. In 1997, as well as during the events in 1989, 1991, the freshwater content in the basin was two times higher (4–5%) with respect to the years without aggregates from 1992 to 1998 (<2%). The biological response to these physical forcings was an intense phytoplankton bloom in the western low salinity waters in July 1997 (O 2sat. up to 206%, Chlorophyll-a (Chl-a) up to 7.64 μg dm −3), a common feature in all years with mucilage. It determined a strong increase of dissolved organic nitrogen (DON, +100%) and phosphorus (DOP, +39%) in the most productive western waters of the basin, which constitute available material for mucilage formation. Nutrient content in the upper waters shifted in concomitance to mucilage appearance, from a PO 4-scarce condition in spring (Si/N/P=225:105:1), to a rather severe dissolved inorganic nitrogen (DIN) and SiO 4-depletion in summer (Si/N/P=21:13:1). This drastic change can stimulate exudate hyperproduction and/or modify bacteria degradation capacity of organic matter. As a consequence of degradation of sinking aggregates, in summer 1997 strong hypoxic conditions were reached in the deeper waters (down to 55% in most of the profile, down to 16% in the western area). Increasing AOU/DIN (up to 143) and AOU/PO 4 (up to 2500) ratios indicated a progressive nitrogen and phosphorous impoverishment in aged aggregates.

Do mucilage events influence pico- and nanoplankton size and structure in the Adriatic sea?

Temporal changes in abundance and biomass of picophytoplankton, heterotrophic pico-eukaryotes, and nanoplankton assemblages were investigated along a transect crossing the Adriatic Sea, from the Italian to the Croatian coast. This 15-months (June 1999–August 2000) investigation allowed comparing microbial parameters during summer 1999 (year without mucilage) and summer 2000 when a major mucilage event occurred. Pico- and nanoplankton assemblages displayed significant differences between the 2 summer periods. The main differences can be summarized as: (i) presence of cyanobacterial blooms (up to 108 cells l−1) in summer 2000, absent in summer 1999; (ii) an increasing fraction of heterotrophic pico-eukaryotes (up to 5.0 × 106 cells l−1) and heterotrophic nanoplankton (size 2–5 µm) during mucilage event; (iii) a reduced abundance of small-sized (2–3 µm) phototrophic nanoplankton in summer 2000. Changes in community structure were signals of changes in trophic condition of the system, which resulted in a competitive advantage for small sized pico- and nanoheterotrophs. Data presented here indicated that mucilage events are associated with changes in microbial community structure and functioning in ambient water and induced the amplification of 3-step microbial food chain. The potential use of the heterotrophic pico-eukaryotes for describing alterations of the trophic pathways during mucilage events is discussed.

Fatty acid and phytoplankton compositions of different types of mucilaginous aggregates in thenorthern Adriatic

The biomarkers (fatty acid proportions and ratios characteristic of phytoplankton and bacteria) and phytoplankton species in small (0.5‐2.0 cm) and large (0.5‐5 m) marine aggregates were determined in samples collected in the northern Adriatic Sea, during a mucilage event in 1997, as well as in 1993, 1994 and 1998, when events were not observed. Types of aggregates were identified according to various biomarker relationships, particularly those related to bacterial and phytoplankton activities and changes in the diatom species composition. Aged mucilaginous aggregates (in summer 1997) showed fatty acid proportions (16P/18P, 3.9‐7.7) characteristic of the highest phytoplankton activities, and also showed the highest bacterial fatty acid proportions (13.3‐17.1%) and ratios (C15:br/C15:0, 4.4‐6.0). They showed an different diatom community (dominated by Cylindrotheca closterium) from that in surrounding waters. These characteristics suggest a continuous renewal of the aggregate organic matter, supporting the hypothesis that aggregate is a ‘selfsustaining’ community. In contrast, both freshly formed marine snow, dominant during periods without mucilage events, and freshly formed mucilaginous aggregates presented biomarker proportions and ratios similar to those of suspended matter (bacterial fatty acids 3‐6.7%, C15:br/C15:0 1.2‐4.6, 16P/18P 0.4‐4.0), and diatom composition similar to that of the ambient water. This indicates that marine snow sinks more rapidly than large aggregates, before significant changes can occur in its plankton composition. Other aging aggregate types showed intermediate characteristics, suggesting that their residence times in the water column were still sufficient to develop organic production‐decomposition cycles that modified to various extents their biochemical composition.

Istrian Coastal Countercurrent and its Year-to-Year Variability

In this paper we demonstrate the existence of a descending surface current which usually appears in August along the Istrian coast line and runs counter to the general Adriatic-wide cyclonic flow. This is achieved by computing the relative (with respect to the 30-dbar surface) geostrophic currents as well as absolute currents and net transports from hydrographic data collected monthly to seasonally at six stations of the Po-Rovinj profile in the northern Adriatic. We name it the Istrian Coastal Countercurrent (ICCC). Its mean relative speed is high, 7cms−1(relative currents at the profile on average stay below 5cms−1in winter and below 8cms−1in summer), inducing a net outflow near the Istrian coast.The ICCC was strong in 1968, 1977, 1983, 1984, 1988, 1989, 1991, 1992, 1996 and 1997, weak or absent in 1971–1973, 1982, 1986, 1993–1995. Whenever the ICCC was well pronounced, specific dynamic and hydrographic features—pointing to a possible existence of an anticyclonic gyre off Rovinj—were observed. The existence of the ICCC is confirmed by direct current measurements, which show that the descending coastal current was intense in 1977 and 1983, and that it did not develop in 1976, 1978 and 1987.The intensity of the ICCC in August over the 1966–1992 interval was significantly correlated to the air–sea heat flux observed with the −7 months time lag in combination with the Po River discharge rate with the −1 month time lag and to the sea surface temperature recorded in the area with the −5 months time lag. On the correlation basis a weak or non-existent ICCC is hindcasted for 1976, 1978 and 1987. It seems that high intensity of the ICCC coincides with oceanographic conditions which favour near anoxia or mucilage events, observed in northern Adriatic in 1977, 1988, 1989, 1991 and 1997.

The role of reduced sulfur species in the coalescence of polysaccharides in the Adriatic Sea

Abstract Massive mucilage events occur in the Northern Adriatic Sea presumably by cytoplasmic excretions from deteriorated diatoms. During three such events in the summer of 1991, 1997 and 1998 the presence of reduced sulfur species (RSS) was determined in samples of macroaggregate using electrochemical methods (in-phase alternating current (AC) and linear sweep voltammetry). The detected levels of sulfur, expressed as equivalent to sulfide concentrations, were about 200 nM. In the same mucilage samples, concentrations of organic matter were determined in the range from 60 to 600 mg/l of total organic carbon (TOC). The physico-chemical properties of organic matter in the macroaggregates correspond to those of polysaccharides of very high molecular mass. Scanning confocal laser microscopy (SCLM) and fluorescent molecular probes for sugars (the lectins concanvaline A (Con-A)) showed the transformation of polysaccharide polymer structure resulting in the formation of very stable filaments and layers after the treatment of mucilage samples with sodium sulfide. Commercial polysaccharides of bacterial and algal origin (xanthan, carrageenans types I and II, dextran-T-500) have been used to simulate macroaggregate formation under laboratory conditions after treatment with sodium sulfide. Raman spectroscopy indicated that for all model polysaccharides used, sulfide interaction occurred, as evidenced by visible change of the O–H stretching region in the vibration spectra of the water molecules. Our data suggest that the aggregated polysaccharides from the Adriatic Sea are: (1) structurally affected by addition of sulfide as was the case for most of the model polysaccharides, and (2) the stabilizing effect of sulfide on the aggregated polysaccharides is due to the formation of sulfur-organic compounds.

Marine optical measurements of a mucilage event in the northern Adriatic Sea

Optical measurements performed during the summer of 1997 from an off‐shore oceanographic tower in the northern Adriatic Sea in the presence of mucilage matter are presented. These measurements include apparent (diffuse attenuation coefficient, reflectance) and inherent (absorption and beam attenuation coefficients) optical properties of the water column at several wavelengths. The presence of mucilage layers (which was confirmed by divers) was associated with unique, very sharp subsurface maxima in the vertical distribution of both apparent and inherent optical properties. These layers were characterized by very high values of the diffuse attenuation coefficient of upwelling radiance and the beam attenuation coefficient, as well as the ratio of the scattering‐to‐absorption coefficients at all wavelengths. The mucilage data were compared to nonmucilage observations and two situations came out regarding surface reflectance ratios: (a) When the mucilage was in (vertically narrow) subsurface layers, mucilage events could not be distinguished from nonmucilage situations; and (b) When the mucilage was more homogeneously distributed throughout the water column and, in particular, approaching the surface, there was a detectable difference with respect to nonmucilage water. However, the data set is small, and the detection capability is based on a small difference, so the variance in nonmucilage water properties may still mask the properties of the mucilage‐contaminated water.

Prediction of Sporadic Mucilaginous Algal Blooms in the Northern Adriatic Sea

In the eutrophic waters of the Northern Adriatic, occasional, large scale mucilage events cause economic and ecological damage. A hypothesis linking biology, hydrography and water chemistry at the mouth of the Po River is proposed to explain the sporadic occurrence of mucilaginous algal blooms. We propose that the formation of stable mucilaginous macroaggregates associated with the blooms occurs in the following temporal sequence: low flow in the Po River leads to nutrient limitation offshore. Mucilage is produced by nutrient limited phytoplankton. Stable hydrographic conditions (which are consonant with low river flows) allow the development of mucous macroaggregates under conditions of low calcium concentration. A subsequent period of hydrographic mixing with the relatively high calcium concentrations in seawater causes stabilisation of the macroaggregates. Seawater circulation then causes the movement of stable mucilage aggregates in a southerly direction parallel to the coast or onto the beaches.

Unusual changes of zooplankton fatty acid composition in the northern Adriatic during the 1991 mucilage event

Fatty acids and pristane/heptadecane (Pr/C17) ratlo were analysed in zooplankton samples collected with a sediment trap in the northeastern Adriatic Sea from April 1991 to June 1992 in 2 to 6 wk intervals. Saturated (SAT; 22 to 68% of total fatty acid mass fraction), monounsaturated (MONO; 20 to 39'0), and polyunsaturated (PUFA; 6 to 49%) fatty acid proportions, total fatty acid mass fraction (2.3 to 46.5 mg g-I dry wt), and Pr/CII ratio (0.5 to 27) varied significantly during the investigated period. Unusually low fatty acid mass fractions (2.3 to 5.1 mg g.' dry wt), PUFA proportions (6%) , and Pr/C17 ratio [< l ) measured In summer 1991 were ascribed to a drastic zooplankton starvation due to a mucilage event, a phenomenon that can occur periodically (every 10 to 50 yr) in the region. Low PUFA proportions (12%) were also measured in winter/early spring 1992, but the C14:O and C16:l proportions, and Pr/C,, ratio (9 to 21) exhibited their highest values of the investigated period. This indicated more favourable feeding conditions and significant phytoplankton contributions in the zooplankton diets than in summer 1991. The highest total mass fraction (12.2 to 46.5 mg g-' dry wt) and PUFA proportions (30 to 49 %) were observed during spring and autumn, when the food supply was probably sufficient and diverse. The olelc acid (C18:lng) proportions (12 to 22%) in the northeastern Adriatic were higher than in other marine regions.

Effects of a mucilage event on the Mediterranean gorgonianParamuricea clavata. II ‐ Population recovery after two years

Abstract A population of the Mediterranean gorgonian Paramuricea clavata was heavily damaged by a mucilage coverage of colonies at the end of summer 1993, and suffered a relatively high mortality. Gorgonian population structure before (summer 1992), and two years after (summer 1995) the mucilage event was compared in order to assess the recovery of the population. Despite gorgonian mean density being practically identical on both dates, suggesting that the local population had fully recovered its effectives, the demographic structure of the P. clavata population showed changes from one sampling date to the other. The most striking effect of disturbance due to mucilage was the rejuvenation of the population: in 1995, juveniles constituted 26.1% of the population, while in 1992 they were only 4.2%.

Effects of a mucilage event on the Mediterranean gorgonianParamuricea clavata. I ‐ Short term impacts at the population and colony levels

Abstract A Paramuricea clavata population thriving between 29 and 39 m depth on a shoal at the northern entrance to the Strait of Messina (Tyrrhenian Sea) was affected by mucilage coverage at the end of summer 1993. Mucilage became entangled in projecting branches and necrotized the coenenchyme below, leaving the axial skeleton bare. The entire population was heavily affected, extent of injuries being negatively correlated with size of colonies. Colony size, scale and location of damage were related to successful regeneration after six months. Smaller colonies showed a higher mortality, while colonies with over 50–60% of their total branch length damaged were overgrown by invaders and died. The extent of injuries in the central part of the colony was found to be a critical parameter in determining colony regrowth. A number of injured colonies grew during the six months following the disappearance of mucilage, which shows that their regrowth was apparently not influenced by stress due to mucilage.

Statistical inferences about the mucilage events in the Adriatic Sea, with special reference to recurrence patterns and claimed relationship to sun activity cycles

The secular hazard rate of mucilage events in the Northern Adriatic Sea appears to have remained constant over the last 120 years. The recurrence pattern can be described by either the exponential or the more general Weibull distribution model. There is a 50% probability that the next arrival of a mucilage event lies within a time period of 4–5 years, and a 90% probability for a mucilage event to return within 13–14 years or less. However, most events will be of local importance only and less severe than the 1988/1991 events. No statistically significant relationship between mucilage events and the 11-year sun activity cycles could be established. Increasing eutrophication and pollution do not appear to have a direct bearing on mucilage recurrence.

Mucilages in Italian seas: the Adriatic and Tyrrhenian Seas, 1988–1991

Mucilage events, i.e. the accumulation of a gelatinous material at and below the water surface during summer months, have been known to occur at irregular intervals in the Northern Adriatic Sea for over 200 years, but events of the severity of those of 1872 and 1905 were less noted later in this century. The phenomenon reappeared in 1988 and in the years that followed. In 1991 it was also noted in the Tyrrhenian Sea. Our studies cover the events of 1988–1991 in the Northwest Adriatic and some aspects of the Tyrrhenian Sea. The results presented in this paper provide additional insight into the problem. In general, it can be stated that while we have a fair understanding of the phenomenological and dynamic aspects, and of the environmental consequences of the events, we know little about the causal factors which trigger and maintain the process.