Benthic–pelagic coupling in the oligotrophic Cretan Sea

Abstract

During the EU-MAST2 project CINCS (pelagic-benthic Coupling IN the oligotrophic Cretan Sea), sediment community oxygen consumption (SCOC) was measured during the winter and summer of 1995. Satellite CZCS images showed a different phytoplankton biomass in the surface water during these two periods. SCOC was measured in-situ with a benthic lander at depths ranging from 40 to about 1600 m. In conjunction to the SCOC measurements, microbial biomass and chlorophyll content of the surface layer of the sediment were also determined. SCOC, microbial biomass and the chlorophyll concentration displayed similar spatial and temporal trends, i.e. a steady decrease with increasing water depth in both seasons, with lower values occuring below 40 m depth during the summer. In winter SCOC ranged from 438 μmol m⁻² h⁻¹ at 40 m depth to 37 μmol m⁻² h⁻¹ at 1570 m which is equivalent to carbon mineralisation varying from 107 to 9 mg C m⁻² d⁻¹, respectively. SCOC values in the summer were about half the winter values except at the 40 m station where the opposite was found. Short-term deployments of a sediment trap 4 m above the sea floor showed diminished vertical fluxes of phytodetritus were lower in summer at all stations including 40 m depth. It is argued, partially on the basis of oxygen microprofiles, that the enhanced sedimentary concentration and decreased vertical flux of chlorophyll in summer at 40 m are primarily caused by benthic primary production. The results from the other stations show that in an highly oligotrophic sea such as the Cretan Sea, pelagic-benthic coupling exists although the amplitude of the seasonal signal is small. Moreover, the bathymetric trends in benthic microbial biomass and SCOC indicate that there is no substantial horizontal transport of labile organic material down the slope.

Introduction

In general, heterotrophic benthic organisms that live in subtidal sediments thrive on organic matter (OM) that has been produced in, and subsequently exported from, the photic zone. The coupling between processes on the seafloor and deposition events has been extensively reviewed by Graf (1992) who provided (conceptual) models depicting the seasonal sequence of events for a shallow boreal habitat (Kiel Bight) and a deeper one in the Norwegian Sea. In both cases there was a rapid and short-lasting increase of the sediment community metabolism (measured as heat production or O₂ consumption) in response to a high quality food pulse as deduced from C/N and chlorophyll/C ratios. This rapid reaction was attributed mainly to small organisms (bacteria, protozoans) whose biomass increased through cell growth and division (Meyer-Reil, 1983). Observations on the coupling of temporal cycles in the benthos and in the water column have now been gathered from a variety of marine environments including several from the deep sea (Pfannkuche, 1993, Lampitt, Raine, Billett & Rice, 1995, Gooday, Pfannkuche & Lambshead, 1996) but relatively few from oligotrophic environments (e.g. Sayles, Martin and Deuser (1994)). Primary production in oligotrophic areas is generally dominated by small planktonic cells, whose organic production is recycled within the complex pelagic microbial food web (e.g. Fenchel (1988), Riegman, Kuipers, Noordeloos and Witte (1993) and Legendre and Le Fèvre (1995)). In addition these small algal cells have very much lower intrinsic sinking rates (Smayda, 1970). All these characteristics serve to minimize exports to deeper waters so that seasonal signals in the benthos will be faint and bentho-pelagic coupling less evident. Hence, activity and abundance of benthic organisms living in oligotrophic, stratified waters are expected to show little if any seasonal variation.

We studied the temporal variation in the standing stock and metabolic activity of benthos on the shelf and slope in one of the world's principal oligotrophic seas, the Cretan Sea in the E-Mediterranean (Fig. 1). Annual primary production in the Aegean Sea is estimated to be 20 g C m⁻² and is likely to be even lower in its southern region (Dugdale & Wilkerson, 1988, Ignatiades, 1998). Phosphorus limitation is one of the major causes for the oligotrophy observed in the Aegean Sea (Krom, Kress, Benner & Gordon, 1991, Tselepides & Eleftheriou, 1992, Ignatiades, Georgopoulos & Karydis, 1995). Moreover, the relatively high water temperatures throughout the total water column in the Mediterranean, can be expected to result in very intensive bacterial degradation of sinking OM (Legendre & Le Fèvre, 1995), and so as the depth increases the residual OM will be increasingly refractory. This will contribute to a reduction in seasonality especially in deep waters. Earlier studies in the Cretan Sea by Tselepides and Eleftheriou (1992) and Danovaro, Fabiano and Della Croce (1993) have revealed that the biomass of macrofauna and meiofauna is very low, which is consistent with the oligotrophy of the water column. The same seems to hold for bacterial biomass reported by Danovaro, Fabiano, Albertelli and Della Croce (1995) when compared to the deep-sea data compiled by Tietjen (1992).

In the present study the biomass of selected benthic size groups (macrobenthos, heterotrophic nanoflagellates and bacteria) were determined during two seasons putatively representing the times of maximum and minimum water column productivity. We selected bacteria and heterotrophic nanoflagellates to represent the responses of the microbial component of the benthic community to varying food supply in the study area. The importance of the role of bacteria in mineralization of sedimentary organic matter has been well established (e.g. Rowe and Deming (1985)). Heterotrophic nanoflagellates are important grazers of bacteria in marine sediments and the abundance of both groups is governed by the supply of organic matter, as has been demonstrated in both field and experimental studies (Bak, van Duyl, Nieuwland & Kop, 1991; Bak, van Duyl & Nieuwland, 1995; van Duyl, Bak, Kop, Nieuwland, Berghuis & Kok, 1992). Because of their body size and morphological diversity macrofauna are considered to be amongst the first groups of organisms to benefit from settling organic particles and hence to show a response to fresh inputs of detritus. We used oxygen uptake of whole sediment cores as a comparable measure for the metabolic activity of the benthic communities (SCOC (Smith, Kaufmann & Baldwin, 1994; Rowe, Boland, Phoel, Anderson & Biscaye, 1994)). Because of the oligotrophy, oxygen will probably be the major oxidant for the OM in the Cretan Sea sediments (Middelburg, Vlug & Van der Nat, 1993). Helder (1989b) has shown this to be true for the Gulf of Lions (W-Mediterranean) where conditions are more eutrophic, judging from the phytoplankton biomass. In conjunction to SCOC, we have used porewater oxygen micro-profiles to estimate the penetration depth of O₂ and the diffusive O₂ flux into the sediment, both of which vary with the amount of available labile OM (Revsbech, Sørensen & Blackburn, 1980). Finally, we measured the flux of chlorophyll-a to the benthic boundary layer and its concentration in the surface sediment. Its relatively high rate of decay (Sun, Lee & Aller, 1993) and its unique origin, make chlorophyll-a a useful tracer of recent deposited and relatively fresh surface-derived OM (Smith et al., 1996, Thiel et al., 1989).