Three oceanographic campaigns were carried out in the Gulf of Genoa, Ligurian Sea (NW Mediterranean), during summer 2002 and spring and late autumn 2003, to study organic matter production (reflected in chlorophyll-a concentrations) and recycling (assessed by means of ectoenzymatic activity evaluations and bacterial abundance and biomass determinations) in the water column down to about 1000 m. The Liguro-Provenc¸al frontal area is known to fertilise the surface waters of the central Ligurian Sea by the intrusion of intermediate waters that are rich in inorganic nutrients. This dynamic process increases the chlorophyll-a concentrations in the southern part of the Gulf (up to 0.57 µg l-1 during spring and up to 0.59 µg l-1 during summer at the deep chlorophyll maximum, placed between 50 and 70 m, ranging in the other sampling areas from 0.08 to 0.28 µg l-1 and 0.12 to 0.25 µg l-1 during spring and summer, respectively). Nevertheless, the autotrophic biomass is rarely higher than the bacterial one (the ratio between primary and bacterial biomass, integrated for the first 200 m, ranged from 0.9 to 1.5 in the southern stations and from 0.3 to 0.8 in the other stations during spring, and from 0.2 to 0.6 at all stations during the other seasons), confirming the relevant role of pico-heterotrophs and organic matter recycling. The transfer of organic matter to depth, driven by downwelling processes and particle sinking, enhances the recycling processes, as indicated by the increase in ectoenzymatic activity in the subsurface layers. A comparison with other studies of the open Ligurian Sea revealed higher proteolytic activities in the deeper layer of the southern zone of the Gulf of Genoa and at the canyon heads (the integration for the layer 200–1000 m showed values up to 3.2, 3.0 and 5.4 nmol l-1 h-1 for leucine aminopeptidase in spring, summer and autumn, respectively), with high perbacterial cell values (on average the values ranged from 163 to 345 amol cell-1 min-1 for autumn and summer, respectively), suggesting a particular adaptation of these microorganisms to the exploitation of organic polymers. Moreover, the turbidity profiles suggested the irregular resuspension of organic and inorganic materials embedded in the bottom sediments next to canyon heads, leading to a significant stimulation of h-glucosidase activity in particular (integration from 200 to 1000 m ranging from 0.06 to 0.14 nmol l-1 h-1, while the other stations showed values on average of 0.02 nmol l-1 h-1 in spring and autumn and 0.04 nmol l-1 h-1 during summer). These processes enhance the recycling and benthic–pelagic coupling in an area where biological processes were thought to be mainly confined to the surface layer.
Ectoenzymatic activity and its relationship to chlorophyll-a and bacteria in the Gulf of Genoa (Ligurian Sea, NW Mediterranean).
MISIC, CRISTINA;FABIANO, MAURO
2006-01-01
Abstract
Three oceanographic campaigns were carried out in the Gulf of Genoa, Ligurian Sea (NW Mediterranean), during summer 2002 and spring and late autumn 2003, to study organic matter production (reflected in chlorophyll-a concentrations) and recycling (assessed by means of ectoenzymatic activity evaluations and bacterial abundance and biomass determinations) in the water column down to about 1000 m. The Liguro-Provenc¸al frontal area is known to fertilise the surface waters of the central Ligurian Sea by the intrusion of intermediate waters that are rich in inorganic nutrients. This dynamic process increases the chlorophyll-a concentrations in the southern part of the Gulf (up to 0.57 µg l-1 during spring and up to 0.59 µg l-1 during summer at the deep chlorophyll maximum, placed between 50 and 70 m, ranging in the other sampling areas from 0.08 to 0.28 µg l-1 and 0.12 to 0.25 µg l-1 during spring and summer, respectively). Nevertheless, the autotrophic biomass is rarely higher than the bacterial one (the ratio between primary and bacterial biomass, integrated for the first 200 m, ranged from 0.9 to 1.5 in the southern stations and from 0.3 to 0.8 in the other stations during spring, and from 0.2 to 0.6 at all stations during the other seasons), confirming the relevant role of pico-heterotrophs and organic matter recycling. The transfer of organic matter to depth, driven by downwelling processes and particle sinking, enhances the recycling processes, as indicated by the increase in ectoenzymatic activity in the subsurface layers. A comparison with other studies of the open Ligurian Sea revealed higher proteolytic activities in the deeper layer of the southern zone of the Gulf of Genoa and at the canyon heads (the integration for the layer 200–1000 m showed values up to 3.2, 3.0 and 5.4 nmol l-1 h-1 for leucine aminopeptidase in spring, summer and autumn, respectively), with high perbacterial cell values (on average the values ranged from 163 to 345 amol cell-1 min-1 for autumn and summer, respectively), suggesting a particular adaptation of these microorganisms to the exploitation of organic polymers. Moreover, the turbidity profiles suggested the irregular resuspension of organic and inorganic materials embedded in the bottom sediments next to canyon heads, leading to a significant stimulation of h-glucosidase activity in particular (integration from 200 to 1000 m ranging from 0.06 to 0.14 nmol l-1 h-1, while the other stations showed values on average of 0.02 nmol l-1 h-1 in spring and autumn and 0.04 nmol l-1 h-1 during summer). These processes enhance the recycling and benthic–pelagic coupling in an area where biological processes were thought to be mainly confined to the surface layer.
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