Species-specific phytoplankton sedimentation in ...

Journal of Plankton Research Vol.20 no.ll pp.2053-2070, 1998

Species-specific phytoplankton sedimentation in relation to primary production along an inshore-offshore gradient in the Baltic Sea Petra Tallberg and Anna-Stiina Heiskanen1 Department of Limnology and Environmental Protection, Section of Limnology, PO Box 27 (E-building), FIN-00014 University of Helsinki and 'Finnish Environment Institute, PO Box 140, FIN-00251 Helsinki, Finland

Introduction

Export production can be denned as the part of the primary carbon produced in the euphotic layer that is exported from the photic zone by sedimentation (Wassmann, 1990). This export production can be measured with sediment traps below the photic zone, and may, at steady state and in pelagic systems, approach new production, i.e. the share of the primary production which is based on external nutrients brought into the pelagic system by physical processes such as mixing or upwelling (Dugdale and Goering, 1967; Eppley and Peterson, 1979; Wassmann, 1990). In shallow coastal environments, however, the disturbing influence of the coast (advective transport of allochthonous material) and the sediment (resuspension of particulate material) make measurements of the primary particle export from the photic zone difficult (Smetacek, 1980; Blomqvist and Larsson, 1994; Heiskanen and Leppanen, 1995). The material collected in the traps is, undoubtedly, a source of organic material for the benthos (e.g. Smetacek, 1984), but may not consist altogether of new or even exported planktonic production. The proportion of the primary production that settles out of the photic zone in the Baltic Sea varies according to the distance from the coast and during the seasonal cycle (Smetacek, 1984; Heiskanen and Kononen, 1994). To use only the mass flux, or the flux of carbon or nitrogen, as an estimate of the export from the production zone loses important information contained in the sinking particles themselves (e.g. Passow and Peinert, 1993). Most autochthonous production in pelagic water ecosystems is initially channeled through phytoplankton cells, but © Oxford University Press

2053

Downloaded from plankt.oxfordjournals.org by guest on July 6, 2011

Abstract. The temporal and spatial variability in the quality and quantity of settling phytoplankton material in relation to concurrent primary production was studied using sediment traps at three coastal stations from a semi-enclosed bay (Pojo Bay) through the outer archipelago to the open Gulf of Finland. The flux of settling phytoplankton was high (9.3 g C m"2 period"1) in Pojo Bay, especially in spring, and lower in the archipelago (8.1 g C m~2 period"1) and open-sea area (5.2 g C m"2 period"1), although the primary production followed the opposite pattern. A large influx of allochthonous material into Pojo Bay in spring brought allochthonous phytoplankton cells into the traps, but limited primary production. Diatoms were the most abundant settled phytoplankton at all stations, but the species composition varied between Pojo Bay (Aulacoseira spp., Rhizosolenia minima) and the outer stations (Skeletonema costatum, Chaetoceros spp.). At the outer stations, migrating dinoflagellates (Peridiniella catenate) comprised part of the settling material in spring. The high settling flux of the cyanophyte Aphanizomenon flos-aquae is discussed. The species composition of the phytoplankton assemblage influenced the proportion of the total organic carbon sedimentation that consisted of phytoplankton carbon.

P.Tallberg and A.-S.Heiskanen

the extent to and form in which this production settles out of the photic zone vary, making the characteristics of the sinking phytoplankton informative regarding both the functioning of the production system and the coastal and profundal influence (Wassmann, 1990; Passow and Peinert, 1993). The temporal and spatial variability in the quality and quantity of settling phytoplankton material and its relationship to the concurrent primary production were studied along the coastal gradient from a semi-enclosed bay (Pojo Bay) through the inner archipelago to the open Gulf of Finland (Baltic Sea). The study was part of the Pojo Bay project, which was initiated in 1992, following alarming signs of accelerating eutrophication and an increasing oxygen deficit in the bottom water in the firth-like Pojo Bay (southern coast of Finland). Intensive physical, chemical and biological sampling was carried out from March to November in order to understand the reasons for the deterioration of the water quality in the bay [see Stipa (1996) and Heiskanen and Tallberg (1998) for details]. Method

2054


The downward flux of material out of the photic zone was measured with sediment traps at three stations in a coastal area in the Gulf of Finland, Baltic Sea (Figure 1). The innermost station was situated in the semi-permanently stratified firth-like Pojo Bay (station V), which is separated from the rest of the archipelago by a 5-m-deep sill. The trap was moored at 15 m depth at the deepest (42 m) site in the bay. The second station (Storfjarden, station XII) was located in the outer archipelago region at the northern end of a narrow depression leading from the open Gulf of Finland to the inner archipelago zone. This trap was placed at 20 m depth (total depth 38 m). The outermost station was at Storgadden (station P3), a large basin outside the archipelago which is influenced by water masses from both the inner archipelago, the open Gulf of Finland and the Northern Baltic proper. The trap was moored at 20 m depth (total depth 50 m). The hydrography and nutrient dynamics of the study area have been described in more detail by Niemi (1975) and Haapala (1994), and the sedimentation dynamics by Heiskanen and Leppanen (1995) and Heiskanen and Tallberg (1998). Water column samples were taken weekly between 10 March and 26 May, and thereafter every other week until the end of November 1992. The sampling depths were 0, 2.5, 5, 7.5, 10, 12.5, 15, 20, 30, 40 and 50 m (where applicable). Salinity (Practical Salinity Scale) and temperature were measured with CTDplus 100 (Sis-Fieldsoft). Inorganic nutrients (NO2, NO3, NH,, PO4) were analyzed manually 1-3 h after sampling according to Grasshoff et al. (1983). Chlorophyll a samples were filtered on Whatman GF/F glass filters and analyzed spectrophotometrically with ethanol as a solvent according to the recommendations of the Baltic Environmental Protection Commission (Helsinki Commission, 1988). The primary production was measured with the 14C technique (Steemann Nielsen, 1952) as described in more detail by Tuomi et al. (1998). Samples were taken weekly (March-May) and every other week (May-September) from 0.2,2.5 and 5.0 m depth at all three stations. All samples were incubated in situ at station XII. Cylindrical sediment traps (diameter 10 cm, aspect ratio 10:1) were used to

Species-specific phytoplankton sedimentation

Sweden


60°00'

59045" Fig. L The study area. V is the sampling station in Pojo Bay, XII is Storfjarden in the inner archipelago and P3 is Storgadden in the outer archipelago/open Gulf of Finland.

collect settling material. Concentrated formaldehyde was added to the bottom of the cylinders through an internal diffusion chamber (the maximal concentration in the collected material was 1.87%). The traps were emptied weekly from March to May and every second week from June to November. Upon retrieval, a constant volume of water from the upper part of the cylinders was discarded, and the remaining suspension (-4 1) was collected through a tap at the bottom of the cylinder and measured. The sample was homogenized before subsamples for chemical and biological analyses were taken. Samples for particulate organic carbon (POC) were filtered in duplicate on pre-combusted Whatman GF/F glass fiber filters, after which mesozooplankton 2055

P.Tallberg and A.-S.Heiskanen

swimmers (copepods and cladocerans) were removed from the filters under a stereomicroscope. The samples were thereafter analyzed with a LECO CHN analyzer. Subsamples for phytoplankton counts were preserved with Lugol's solution and the dominant taxa were counted according to Utermohl (1958). A minimum of 500 taxonomic units were counted from each sample. Cell volumes and carbon contents were calculated according to Edler (1979). Results The temperature and salinity patterns at the sampling stations are shown in Figure 2. At station V in Pojo Bay, the water column was thermally stratified from


50 m Mar. April May

June

July Aug. Sept Oct

Nov.

Fig. 2A. Temperature (°C) at the three sampling stations from March to November 1992.

2056

Species-specific phytoplankton sedimentation

the beginning of May to the end of October. A salinity gradient of 2-4 persisted at 10 m depth for most of that period. At stations XII and P3, the temperature stratification lasted from June to September. Upwelling events in early May and June were observed as increases in the deep-water salinity. Owing to regulation of the Svarta river, its discharge into Pojo Bay during the summer followed a very regular pattern with high values in spring and autumn, and low, constantflowduring the summer (Figure 3). The Secchi depths were low at station V in Pojo Bay in spring and late autumn (