Paolo Magni

Sediment grain size and organic carbon distribution in the Cabras lagoon (Sardinia, Western Mediterranean)

The aim of this work was to investigate the spatial variability of sediment bulk properties in the surface sediments as well as down-core trends, and to assess the relationship between sediment distribution and levels of total organic carbon (TOC), in the Cabras lagoon, Sardinia. Grain size distributions and TOC contents were measured in the surface sediments (0–2 cm horizon) in a grid of 31 stations. Vertical profiles along the 0–24 cm depth horizon were also measured in three stations. In the superficial horizon, TOC content was very high, up to 43 mg g−1, with a mean of 33 mg g−1. Core profiles showed a marked reduction of TOC content with depth to ∼20–25 mg g−1 and a concomitant shift in particles size towards a sandier composition (mud content decreasing from 95–100% to 70–85%) at 3–7 cm core depth. Total organic carbon contents in the uppermost layers were associated with the grain size fraction <8 µm. The results suggest that a major change in the sedimentary regime of the lagoon, associated with internal trapping and re-distribution of organic C-bounding fine particles, has been occurring in the last few decades. The cause would appear to be the construction of a dam at the lagoon mouth rising up to the high tide level in order to maintain a constant lagoonal water level.

The Effect of a Tidal Cycle on the Dynamics of Nutrients in a Tidal Estuary in the Seto Inland Sea, Japan

A 24 hour time series survey was carried out during a spring tide (tidal range ca.2 m) of May 1995 on a tidal estuary in the Seto Inland Sea, Japan, in the context of an integrated program planned to quantify the dynamics of biophilic elements (carbon, nitrogen and phosphorus) and the roles played by the macrobenthos on the processes. Three stations were set along a transect line of about 1.4 km, which linked the river to the rear to the innermost part of the subtidal zone. Every hour, at each station, measurements were made of surface water temperature, salinity and dissolved oxygen concentration, and surface water was collected for the determination of nutrients [NH4+−N, (NO3−+NO2−)−N, PO43−−P and Si (OH)4−Si]. During the ebb flow, riverine input of silicate and nitrate+nitrite significantly increased the concentrations of both the intertidal and the subtidal stations. Conversely, during the high tide, river nutrient concentrations were lowered by the mixing of fresh water with sea water. As a result, best (inverse) correlations were found at the river station for salinity against silicate (y=-2.9 Sal.+110.7,r2=0.879) and nitrate+nitrite (y=-1.3 Sal.+48.4,r2=0.796). In contrast, ammonium nitrogen concentrations were higher at intermediate salinities. Indeed, no significant correlation was found between salinity and ammonium. The effect of the macrobenthos, which is abundant on the intertidal flat, is discussed as a biological component that influences the processes of nutrient regeneration within the estuary. The effect of the tidal amplitude is an important one in determining the extent of the variations in nutrient concentrations at all three stations, which were stronger between the lower low tide and the higher high tide.

Semidiurnal Dynamics of Salinity, Nutrients and Suspended Particulate Matter in an Estuary in the Seto Inland Sea, Japan, during a Spring Tide Cycle

The physical and chemical variability of the water column at subtidal station of an estuary in the Seto Inland Sea, Japan, was studied over a 24-hour period during a spring tide (tidal range ca. 2 m) in May 1995. Surface water and several depths through the water column were monitored every one and two hours, respectively. At each occasion, water temperature, salinity and dissolved oxygen concentration were measured and water samples were collected for the determination of nutrients and suspended particulate matter (SPM). Disruptive changes in the physical and chemical characteristics of the water was produced by the tidal cycle and the mixing of water masses of different origin. These changes were highly significant both spatially and temporally, yet with varying effects on physical parameters, nutrients and the different components of SPM. Significant differences in nutrient concentrations were also observed when the data-set was divided into ebb and flood components, irrespective of the depth. Nitrate and nitrite rose to 1.8 times higher during the flood. Spatial differences of SPM were less marked than those of nutrients, only particulate organic carbon (POC) being significantly higher at the surface than in the intermediate and the lower layer. Both POC and pheopigment concentrations increased markedly through the water column, being highest shortly before the lower low tide. In contrast, suspended solid (SS) content increased sharply after the lower low tide (>40 mg l−1) and this coincided with a marked decrease of the C/SS content (<20 mg g−1). The lagtime between POC and SS tidal transport was caused by particle resuspension from the exposed intertidal sediments as the tidal level rose, and particle transport selection in relation to the tidal state.

Macrofaunal community structure and distribution in a muddy coastal lagoon

Macrofaunal communities and specific bulk properties of organically enriched sediments were investigated in the coastal lagoon of Cabras (Sardinia, western Mediterranean) on a grid of twenty-nine sampling stations, in spring 2001. Species composition and community structure indicated poor and heterogeneous macrofaunal communities, characterized by few predominant taxa typical of degraded environments, such as Polydora ciliata, Tubificidae nc and Neanthes succinea, distributed differently in some areas of the lagoon. Sediments were homogeneously muddy, with a mean silt + clay content of 93%. The analysis of specific grain size intervals within the mud fraction, however, highlighted a marked spatial variability of sediment particle distribution. Simple associations included a positive relationship of both Ficopomatus enigmaticus and Corophium sextonae, patchily distributed along the shores, with sediment sorting (σ), an index of sediment selection due to hydrodynamic energy. By contrast, inner areas, characterized by a major accumulation of finer particles (and organic matter), with a sediment mean size (Φ) up to 8.2 phi, were least populated. The results suggest the existence of an early stage of faunal succession which might be related to an excessive organic content of sediments and the tendency to dystrophic events in the Cabras lagoon.

Water chemistry variability in the lower intertidal zone of an estuary in the Seto Inland Sea, Japan: seasonal patterns of nutrients and particulate compounds

From April 1994 to April 1996, we carried out monthly surveys on the physical and chemical variability of ebbing water at two intertidal stations near the emerged tidal flat and surface water at a nearby subtidal station, in a tidal estuary of the Seto Inland Sea, Japan. The fresh water runoff was a major source of new nitrogen (nitrate + nitrite = −2.1 × salinity + 74.3, r2 = 0.76, p < 0.001, at the inner intertidal station). In contrast, the absence or weak correlation of ammonium, phosphate and silicate with salinity in ebbing water highlighted the importance of in situ biologically-mediated processes, including the excretory activity of intertidal dominant bivalves (Ruditapes philippinarum and Musculista senhousia) and primary producer nutrient uptake. Among suspended particulate matter (SPM), the negative correlation of Chl a with salinity (p< 0.001) suggested that an upper intertidal zone, where high microphytobenthic assemblages occur, may represent a considerable source of microalgal biomass. Whereas, a stronger correlation of POC with pheo-pigments (r2 ≥ 0.92) than with Chl a (r2 ≤ 0.44), a low fraction of living phyto-Carbon (ca. 5%) and a high pheo-pigment/Chl a ratio (ca. 3) indicated a high portion of refractory algal material and a close water–sediment coupling on the flat, in which abundant macrozoobenthos is likely to play an important role. In spite of the strong variability of this ecosystems, this study also demonstrated that highly significant seasonal patterns can be recognized in our study area. However, patterns varied depending on the different nutrient species, SPM and sites. In ebbing water, ammonium concentrations were significantly higher in the cold period (i.e. November–April: water temperature 10.0 ± 4.4 °C), when primary producer biomass decreases, while Chl a and pheo-pigment content were 4.3 and 4.8 higher in the warm period (May–October: water temperature 23.5 ± 4.2 °C), respectively. At the subtidal site, nutrients were 1.9 (silicate)–2.9 (phosphate) and SPM was 1.8 (POC)–2.1 (pheo-pigments) times higher in the warm period. On an annual basis, mean pheo-pigment and POC content was 12–25 times and 4–8 times higher on the intertidal zone than on the subtidal zone, respectively, while nutrient concentrations were within the same order of magnitude. Such a qualitative difference (i.e. nutrients vs. SPM) between sites suggests that particle deposition and/or removal is likely to occur along the estuary, resulting in a limited SPM content at the surface layer of the subtidal site, while nutrients are more directly transported into the surface layer of the subtidal zone by lower salinity water mass intrusion from the intertidal zone, most importantly during the warm period.

Influence of the level of oxygenation in sediment and water on copper bioavailability to marine bivalves: laboratory experiments and translocation experiments in the field

The effects of differences in the level of oxygenation of sediment or water on the condition and copper content of two bivalves, the Baltic clam Macoma balthica and the cockle Cerastoderma edule, were assessed. Specimens from four intertidal flats in the Netherlands and France were compared, translocated and exposed to different levels of oxygen in the laboratory. Cockles showed no significant differences in condition and copper content between animals from light (= more oxygenated) and dark (= less oxygenated) sediments. Baltic clams also showed no differences in condition, but the clams had a higher copper content (concentration as well as body burden) in dark than in light sediments. During the translocation experiments no significant changes occurred. In the laboratory experiments the level of oxygen had no effect on the condition or copper content of the Baltic clam. The only factor affecting the copper content of Baltic clams was the addition of copper to the water or sediment. The copper, organic carbon and silt fraction (< 16 µm) was higher in dark sediments than in light sediments. The copper content in the sediment was positively related to the silt and organic carbon content. We argue that the relation between coloration (= degree of oxygenation) of sediments and the copper content of Baltic clams could be indirect: due to a higher silt fraction and/or organic content at some places on a tidal flat, these places are more hypoxic and therefore darker, whereas simultaneously these places have a higher copper concentration because of more copper-complexing sites (and surface), whereby the higher copper concentration in the sediment relates to a higher copper concentration in the clams.