Norberto Della Croce

Dynamics of meiofaunal assemblages on the continental shelf and deep-sea sediments of the Cretan Sea (NE Mediterranean): relationships with seasonal changes in food supply

Abstract Quantitative information on metazoan meiofaunal abundance and biomass was obtained from three continental shelf (at 40, 100 and 200 m depth) and four deep-sea stations (at 540, 700, 940 and 1540 m depth) in the Cretan Sea (South Aegean Sea, NE Mediterranean). Samples were collected on a seasonal basis (from August 1994 to September 1995) with the use of a multiple corer. Meiofaunal abundance and biomass on the continental shelf of the Cretan Sea were high, in contrast to the extremely low values reported for the bathyal sediments that showed values comparable to those reported for abyssal and hadal environments. In order to explain the spatial and seasonal changes in metazoan meiofauna these data were compared with: (1) the concentrations of ‘food indicators’ (such as proteins, lipids, soluble carbohydrates and CPE) (2) the bacterial biomass (3) the flux of labile organic compounds to the sea floor at a fixed station (D7, 1540 m depth). Highly significant relationships between meiofaunal parameters and CPE, protein and lipid concentrations and bacterial biomass were found. Most of the indicators of food quality and quantity (such as CPE, proteins and carbohydrates) showed a clear seasonality with highest values in February and lowest in September. Such changes were more evident on the continental shelf rather than at deeper depths. On the continental shelf, significant seasonal changes in meiofaunal density were related to changes in the input of labile organic carbon whereas meiofaunal assemblages on the deep-sea stations showed time-lagged changes in response to the food input recorded in February 95. At all deep-sea stations meiofaunal density increased with a time lag of 2 months. Indications for a time-lagged meiofaunal response to the food inputs were also provided by the increase in nauplii densities during May 95 and the increase in individual biomass of nematodes, copepods and polychaetes between February and May 1995. The lack of strong seasonal changes in deep sea meiofaunal density suggests that the supply of organic matter below 500 m is not strong enough to support a significant meiofaunal development. Below 700 m depth >92% of the total biomass in the sediment was represented by bacteria. The ratio of bacterial to meiofaunal biomass increased with increasing water depth indicating that bacteria are probably more effective than meiofauna in exploiting refractory organic compounds. These data lead us to hypothesise that the deep-sea sediments of the Cretan Sea are largely dependent upon a benthic microbial loop.

Meiofauna of the deep Eastern Mediterranean Sea: distribution and abundance in relation to bacterial biomass, organic matter composition and other environmental factors

Abstract Quantitative information on the abundance and biomass of metazoan meiofauna was obtained from samples collected at 15 deep-sea stations in the Eastern Mediterranean Sea (533–2400m). Meiofaunal abundance was compared to bacterial biomass and other environmental factors such as the total sedimentary organic matter content, the concentrations of the main biochemical classes of organic compounds (i.e. proteins, carbohydrates and lipids) and to ATP. To estimate the sedimentation potential of primary organic matter, sediment bound chloroplastic pigment equivalents (CPE) were assayed. Meiofaunal density was very low ranging from 4 ind.10cm−2 (Station A4, 1658m depth) to 290 ind.10cm−2 (Station A12, 636m depth). Nematodes were the numerically dominant taxon (68% of total meiofauna) and were usually confined to the top 6cm of the sediments. Total meiofaunal biomass ranged from 2.78μgC 10cm−2 (Station A4) to 598.34μgC 10cm−2 (Station 15A). There was a significant decrease in the density of metazoan meiofauna with water depth. Bacterial biomass largely dominated the total biomass (as the sum of bacterial and meiofaunal biomass) with an average of 73.2% and accounted for 35.8% of the living biomass (as ATP carbon) whereas meiofaunal biomass accounted only for 6.56%. Bacterial biomass was significantly related to the DNA concentrations of the sediment. A significant correlation between ATP concentration and CPE content was also found. No correlations were found between meiofauna, ATP and CPE, or between meiofauna and bacterial parameters. The significant relationship between meiofaunal density and the ratio of labile organic matter/total organic matter indicates that deep-sea meiofauna inhabiting an extremely oligotrophic environment (such as the Eastern Mediterranean) may be more nutritionally dependent upon the quality than on the quantity of sedimentary organic matter.

Biochemical composition of pico-, nano- and micro-particulate organic matter and bacterioplankton biomass in the oligotrophic Cretan Sea (NE Mediterranean)

Abstract The biochemical composition of different particle size classes (pico-, nano- and micro-particulate matter) and the bacterioplankton biomass were studied over an annual cycle in the Cretan Sea (South Aegean Sea, NE Mediterranean; from 40 to 1540 m depth) to investigate the origin, composition and fate of the suspended particles and to quantify bacterioplankton contribution to organic carbon pools. The oligotrophy of this system was indicated by the extremely low particulate lipid, protein and carbohydrate concentrations (4–15 times lower than in more productive systems). The biopolymeric carbon (BPC as the sum of lipid, protein and carbohydrate carbon) accounted for 80–100% of POC, suggesting the autochthonous origin of the particles. The most evident characteristic of this oligotrophic environment was the dominance of the pico-particles through all seasons, accounting for 43–45% of total carbohydrates, proteins and lipids. The proximate composition of the organic particles revealed the dominance of carbohydrates in all size-classes and highest values of the protein to carbohydrate ratio in the pico-particulate fraction. The relative proportion of the pico-, nano- and micro-particulate carbohydrates, proteins and lipids varied seasonally. The increase in the average particle size from February to September 95, probably as a result of aggregation, appeared to be related to the ‘thermal stability’ of the water column. The analysis of the vertical distribution of the three size classes revealed an increase in the pico fraction and a decrease in the larger components with increasing depth suggesting that nano- and micro-particles were being degraded and fragmented in the deeper water layers. Bacterial densities ranged from 1.1 to 8.8 x 108 cells l−1. Bacterial biomass accounted on average for more than 56% (up to 74%) of BPC and was by far, the most important living component. Bacterial-N accounted for a large proportion (>90%) of the protein nitrogen pool, indicating that almost no particulate detrital N was available for heterotrophic metabolism. Therefore, it is likely that bacteria utilise other sources, such as DOM and inorganic nutrients, to support their growth. The lack in particle variability appeared to be responsible for the rather consistent size structure and biochemical composition of the suspended particulate organic matter in this system.

Seasonal changes and biochemical composition of the labile organic matter flux in the Cretan Sea

Abstract Downward fluxes of labile organic matter (lipids, proteins and carbohydrates) at 200 (trap A) and 1515 m depth (trap B), measured during a 12 months sediment trap experiment, are presented, together with estimates of the bacterial and cyanobacterial biomasses associated to the particles. The biochemical composition of the settling particles was determined in order to provide qualitative and quantitative information on the flux of readily available organic carbon supplying the deep-sea benthic communities of the Cretan Sea. Total mass flux and labile carbon fluxes were characterised by a clear seasonality. Higher labile organic fluxes were reported in trap B, indicating the presence of resuspended particles coming from lateral inputs. Particulate carbohydrates were the major component of the flux of labile compounds (on annual average about 66% of the total labile organic flux) followed by lipids (20%) and proteins (13%). The biopolymeric carbon flux was very low (on annual average 0.9 and 1.2 gC m−2 y−1, at trap A and B). Labile carbon accounted for most of the OC flux (on annual average 84% and 74% in trap A and B respectively). In trap A, highest carbohydrate and protein fluxes in April and September, corresponded to high faecal pellet fluxes. The qualitative composition of the organic fluxes indicated a strong protein depletion in trap B and a decrease of the bioavailability of the settling particles as a result of a higher degree of dilution with inorganic material. Quantity and quality of the food supply to the benthos displayed different temporal patterns. Bacterial biomass in the sediment traps (on average 122 and 229 μgC m−2 d−1 in trap A and B, respectively) was significantly correlated to the flux of labile organic carbon, and particularly to the protein and carbohydrate fluxes. Cyanobacterial flux (on average, 1.1 and 0.4 μgC m−2 d−1, in trap A and B, respectively) was significantly correlated with total mass and protein fluxes only in trap A. Bacterial carbon flux, equivalent to 84.2 and 156 mgC m−2 y−1, accounted for 5–6.5% of the labile carbon flux (in trap A and B respectively) and for 22–41% protein pool of the settling particles. These results suggest that in the Cretan Sea, bacteria attached to the settling particles represent a potential food source of primary importance for deep-sea benthic communities.

Biochemical composition of particulate organic matter and bacterial dynamics at the sediment—water interface in a Mediterranean seagrass system

The seagrass Posidonia oceanica is the most productive system of the Mediterranean Sea. In order to gather information on the temporal and spatial variability of the suspended particulate matter in relation to bacterial dynamics, water samples were collected at 10 cm above the sediments over a period of 13 months in the Prelo Bay (Ligurian Sea, NW Mediterranean). Measurements of seston concentration, as well as the elemental (POC and PON) and biochemical composition (lipids, proteins, carbohydrates and nucleic acids) of particulate matter were carried out to assess the origin, composition and bacterial contribution to the food potentially available in the seagrass system to consumers. Lipids and proteins were the main biochemical classes of organic compounds, followed by carbohydrates. Despite the highly refractory composition of the seagrass leaves, particulate organic matter was mostly composed of labile compounds (69.9% of POC). POM temporal patterns were controlled by current speed at the sediment—water interface that resuspended only small particles largely colonised by bacteria after an intensive process of fractionation and aging. In the seagrass system, the POM appears to be dominated by bacteria (density ranging from 0.7 to 2.5 × 109 cells 1−1, representing more than 48.3% of POC and 68.7% of the biopolymeric carbon, as the sum of lipid, protein and carbohydrate carbon). This feature was characteristic of the seagrass system since much lower bacterial densities were found outside the Posidonia meadow. Bacteria were negatively correlated with the concentration of nitrite and nitrate suggesting a selective utilisation of inorganic nutrients to support their growth.

Microbial Response to Oil Disturbance in the Coastal Sediments of the Ligurian Sea (Nw Mediterranean)

Abstract The effects of the oil contamination (10 April 1991) on microbial assemblages of the Ligurian Sea (northwestern Mediterranean) were investigated on samples collected from January 1991 to January 1992. High hydrocarbon concentrations (up to 214.3 μ g−1 DW) occurred during a phytoplankton bloom and had a significant impact on phytoplankton biomass at the sediment-water interface. Chlorophyll-a concentrations showed an abrupt decline (from 2.49 μg 1−1 in pre-pollution conditions on April 8, to 0.75 μg 1−1 just after the oil contamination on April 22). A concomitant increase of phaeopigment concentration was observed (from 0.37 μ1−1 to 2.24 μg1−1). Oil contamination apparently had a limited effect on microphytobenthic communities. Sedimentary chlorophyll-a increased after oil contamination together with a major peak of phaeopigments. By contrast, increased oil concentrations stimulated a significant bacterial response. Benthic bacteria showed a significant increase in density of all the size classes ...

Biochemical composition and trophic strategies of the amphipod Eurythenes gryllus at hadal depths (Atacama Trench, South Pacific)

Amphipods Eurythenes gryllus were collected at 7800 m depth in the Atacama Trench (South Pacific) for studying their biochemical composition (in terms of proteins, lipids and carbohydrates and fatty acid content) and to gather information on bioenergetic strategies and trophic habits of organisms living in this extreme environment. The effect of long-term formalin storage on the biochemical determinations was also determined. Proteins were the dominant biochemical class of organic compounds (39–53%D.W.), whereas carbohydrates accounted for a very small fraction (1–2%D.W.). Lipid concentrations of E. gryllus accounted for 7–18%D.W. and were much lower than those reported for other deep-sea amphipods. These differences are likely to be more dependent upon food availability in the Atacama Trench rather than upon temperature. Lipid composition of E. gryllus revealed the dominance of monounsaturated fatty acids with polyunsaturated fatty acids accounting for a very small fraction, suggesting that hadal amphipods are higher dependent upon lipid reserves than amphipods inhabiting at shallow depths. The ratio of C18:1Δ9 to C18:1Δ11 was >11 confirming the necrophagous trophic habits of this hadal amphipod.

Intertidal benthic communities of two Chilean coastal islands (Santa María and Mocha, Southeastern Pacific)

Six macrotidal beaches located on two Chilean coastal islands (Santa María and Mocha, SE Pacific Ocean) were investigated in spring 2001 (29 October–3 November) to verify the role of the food supply (in terms of quantity and biochemical composition of the organic matter) on the abundance and diversity of the macro- and meiobenthic communities inhabiting the beaches. Samples of sediment were collected from the intertidal zone of the three beaches of each island investigated. The total organic matter content of the sediment did not differ between the islands, whereas the phytopigment and protein contents were significantly higher on Mocha (2.3±0.9 and 75±21 μ g g−1, mean values of the three beaches±SE, respectively) than on Santa María (0.5±0.2 and 50±16 μ g g−1, mean values of the three beaches±SE). The macro- and meiofaunal assemblages displayed the highest abundances on Mocha (821±223 and 561±194 ind. 10 cm−2, respectively). The abundance of both assemblages was correlated with the quality of the food supply, and significant correlations were observed with the phytopigment content (Spearman-rank R=0.645 p<0.01 and R=0.934 p<0.001 for the macro- and meiofauna, respectively). The results of the present study suggest that food availability may play a key role in structuring benthic communities of the oceanic beaches of Chilean coastal islands. A single observation is not enough to fully understand the real mechanisms that shape the beach communities, however, the snapshot image we collected during this study suggests that the role of food availability in shaping benthic beach communities may be as important as that played by the hydrodynamic conditions.

Zooplankton in the surrounding waters of the Juan Fernández Archipelago

Un estudio para el conocimiento de los estadios larvales de Jasus frontalis (Milne-Edwards, 1836) ha permitido estimar la composicion cualitativa y la estructura cuantitativa del zooplancton en las aguas del archipielago Juan Fernandez.