The benthic foraminiferal response to the 2004 spring bloom inthe western Baltic Sea

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Abstract

The diatom spring bloom and response of the benthic foraminifera was monitored between January and July 2004 in the Kiel Bight, western Baltic Sea. Phytoplankton production, the deposition of organic detritus, and feeding of Elphidium excavatum clavatum and Reophax dentaliniformis were monitored by pigment concentrations and pigment composition in water samples, surface sediments, and foraminiferal protoplasm. The population dynamics of the dominant species, Elphidium e. clavatum, was assessed by using their size distributions. The main spring bloom was triggered by a sudden rise in the three-day average insolation to above 170 W m 2 d 1 and took place during late February. Chlorophyll a concentrations in the protoplasm of Elphidium e. clavatum followed those in the surface water. The chlorophyll a / fucoxanthin ratio was the same as in bacillariophycean diatoms suggesting a “bloom-feeding” behaviour of Elphidium e. clavatum. Alternatively, the pigment composition in Reophax dentaliniformis only occasionally mirrored that of diatoms, indicating prochlorophycean and cyanophycean algae as this species' preferred food source. The size distributions and the abundances of living Elphidium e. clavatum revealed two reproduction events, which were 17 days apart. Both events coincided with an increase in sedimentary chlorophyll a concentrations. The reproduction was preceded by growth and feeding of adult specimens and it was followed by distinct growth phases of young specimens, which lasted a few days only. One reproductive cycle covered the spring bloom and produced many more offspring than the subsequent cycle. The light carbon isotopic signal from diatoms was only transiently recorded in foraminiferal calcite and with a low offset during the spring bloom. The oxygen isotopic composition of foraminiferal calcite mirrors seasonal δ18O-seawater–salinity relationships rather than temperature.

Introduction

Benthic foraminifera from the Baltic Sea have been investigated for many decades (e.g. Rhumbler, 1935). The taxonomy and ecology of benthic foraminifera was described by Van Voorthuysen (1960), Lutze (1965) and Haake (1967). Their general distribution patterns were assessed by Rottgardt (1952), Brodniewicz (1965), Hermelin (1987), Kreisel and Leipe (1989), Frenzel (1996) and Frenzel and Oertl (2002). Carbonate production and biomass of benthic foraminifera were estimated by Wefer and Lutze (1978) and Altenbach (1985). During the 1960s and 1970s, the Kiel Bight in the western Baltic Sea was intensively studied by scientists from many disciplines within the framework of the former Special Research Unit (SFB) 95 of Kiel University (Lutze, 1968a, Lutze, 1968b, Lutze, 1974, Wefer, 1976, Lutze et al., 1983). These researchers found that water turbulence and boundary layers between different water masses affected the surface sedimentary environments and composition of benthic foraminiferal assemblages. Other studies focused on primary production and phytoplankton dynamics, which showed a strong seasonality (Bröckel, 1975, Hällfors and Niemi, 1986). The annual primary production was estimated at 158 g C m 2 yr 1 for 1973 (Bodungen, 1975). Not less than 61% of the organic detritus produced by the phytoplankton in the surface water was deposited in the deep basins (Wefer, 1976, Altenbach, 1985). Five to 15% of the total annual flux of particulate organic matter to the seabed was consumed by benthic foraminifera (Altenbach, 1985). Algal spring blooms and pulsed phytodetritus deposition prevailed in the Baltic Sea (Bodungen, 1975, Eversberg, 1990), which has been suggested to have a significant and short-term influence on benthic foraminiferal assemblages (Wefer, 1976). A rapid response of benthic foraminifera to phytodetritus deposition has been described primarily from deep-sea environments (Gooday, 1988, Gooday, 1993, Gooday and Rathburn, 1999, Gooday and Turley, 1990, Altenbach, 1992, Lambshead and Gooday, 1990). Subsequent studies in fjords on the Swedish west coast, and from the outer shelf of the Bay of Biscay demonstrated that sedimentation of organic detritus from the spring phytoplankton bloom stimulated growth of foraminiferal populations (Gustafsson and Nordberg, 1999, Gustafsson and Nordberg, 2000, Gustavsson and Nordberg, 2001, Langezaal et al., 2006). The seasonality of phytodetritus deposition was also reflected in the stable carbon isotopic composition of benthic foraminiferal tests (Mackensen et al., 1993, Corliss et al., 2002, Filipsson et al., 2004).

Many foraminifera can respond within days to nutrient inputs, and their individual bodymass almost doubles due to rapid food ingestion (Altenbach, 1992, Gooday et al., 1992, Gooday, 1994, Moodley et al., 2000). Such “bloom feeding” may be followed by reproduction (Lee et al., 1969, Gooday, 1988), although to date, the trigger of reproduction has not been sufficiently constrained (Murray and Alve, 2000). For example, a number of studies suggested that benthic foraminiferal reproduction is continuous rather than seasonal (Haake, 1967, Wefer, 1976, Murray, 1983).

The aim of the present study is to investigate the link and timing between seasonal phytodetritus deposition, benthic foraminiferal feeding and reproduction in the western Baltic Sea. Tracking this process is important to better understand the connection between primary production and the benthic foraminiferal assemblage response. The data presented here also help to constrain whether fresh food is used directly in building the foraminiferal tests, thus instantaneously altering their stable carbon isotopic composition.

Section snippets

Hydrographical setting

The western Baltic Sea is characterised by two water masses (Rheinheimer et al., 1989). Low-salinity Surface Water (10 to 19 units) is separated by a boundary layer (the halocline) at 16 to 18 m depth from the higher-saline Kattegat Water (17 to 21 units). Primary production is characterised by a strong spring bloom in February, March and April, with diatoms as the main producers (Rheinheimer, 1996; own observations). The spring bloom is followed by several smaller summer blooms from June to

Sampling

Our investigation was based at the “23.5-m station” of Wefer (1976) in the “SFB 95 Hausgarten” area at 54°32′ N and 10°02′ E (Fig. 1). This site was revisited on 13 daily cruises with R/V Polarfuchs from January to July 2004. Every approach to this site is designated as an individual sampling station and labelled as stations PF1 through PF14. The time intervals were frequent at the onset of the first spring bloom in order to sufficiently cover this important event. By mid March, the sampling

Hydrography

The temperature and salinity measurements at the 23.5 m monitoring site reflect the strengthening of the pycnocline since deep winter mixing in January 2004, and the development of stable summer stratification in April (Fig. 2). The pycnocline fluctuated between 9 and 20 m depth and deepened to below 20 m between the 4th and 11th March 2004 such that the 23.5 m site was intermittently bathed by Surface Water.

The salinity gradient between Surface Water and Kattegat Water was rather continuous in

Relationship between phytoplankton bloom and benthic faunal reaction

High phytoplankton production commenced in the Kiel Bight after a sudden increase in solar radiation on the 19th of February 2004, and it took five days for Surface Water chlorophyll a to reach maximum values. Higher chlorophyll a concentrations were not recorded in Surface Water samples at any later measurement during spring and early summer. It is therefore justified to consider this 11-day period of enhanced productivity the 2004 spring bloom. The cessation of bloom activity was most likely

Conclusions

The 2004 spring bloom in the western Baltic Sea was triggered by a sudden rise in average solar radiation to above a threshold value of 170 W m 2 d 1 on the 19th of February. Five days later, the chlorophyll a concentration in the Surface Water had increased to peak values indicating the spring bloom. The plankton bloom ceased after eleven days and chlorophyll a concentrations in the surface sediment showed a maximum two days following this cessation. This fresh organic matter generated during

Acknowledgments

Michael Spindler and Klaus von Bröckel, Kiel, made valuable suggestions for the concept of this study, and they provided equipment and ship time with R/V Polarfuchs. Wolfgang Kuhnt encouraged JS to resume studies on Baltic Sea foraminifera that were pending at Kiel for 20 years, and he gave access to the reference slides of G.–F. Lutze at the Micropalaeontology Unit, Institute of Geosciences, Kiel University. Karin Lochte, Kiel, permitted to use the HPLC equipment at IFM-GEOMAR, Kiel. Jutta

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