Seasonal variation in δ13C and δ15N of size-fractionated plankton at a coastal station in the northern Baltic proper

Abstract

Seasonal cycles of δ 13 C and δ 15 N in dissolved organic carbon and size-fractionated plankton, ranging from bacteria to the jellyfish Aurelia aurita, were studied during a 1 yr cycle at a coastal station in the Baltic Sea. The observed isotopic changes were found with time lags in all size-fractions of plankton. The δ 13 C showed a bimodal cycle with 2 local maxima, the first coinciding with the spring bloom and the second with the autumn bloom. In δ 15 N, the annual cycle was trimodal with 3 local maxima. The first occurred in connection with the spring bloom, the second in mid-summer and the third was a broad autumn-to-winter maximum. The causes of these patterns are discussed in relation to measured oceanographic variables. In the summer, a depleted nitrogen isotopic signal was propagated through all size-classes of plankton, indicating direct or secondary utilisation of fixed nitrogen from cyanobacteria. The strength of the signal indicated that nitrogen-fixing cyanobacteria are more ecologically important as instantaneous nitrogen sources in the Baltic than previously assumed. Enrichment of δ 15 N in size-classes of plankton was found to be a linear function of logarithmic organism size from 20 to 500 µm, reflecting size-related consumption patterns of marine plankton food-webs. The explanatory power of the linear regression and the enrichment per unit size were stronger in spring and autumn than in the summer, reflecting time lags and diversity in the zooplankton community. The sizespecific approach was found to be a simpler and more appropriate way of analysing trophic isotope enrichment in plankton food-webs than the assumption of a general enrichment factor per trophic level. 203: 47-65, 2000 of phytoplankton, but the pattern was variable between years. Zohary et al. (1994) found, in a long-term study of plankton in Lake Kinneret (1971Kinneret ( to 1992, that δ 13 C was most depleted during winter in both phyto-and zooplankton. The available information suggests a characteristic seasonal cycle, but sample variation has often been difficult to separate from seasonal variation. If the seasonal isotopic cycle is not properly described for organisms at a studied site, time lags and memory effects may interfere with attempts to analyse food-web dynamics and material use. KEY WORDS: Stable isotopes · Food web · Baltic · Cyanobacteria · Seasonal cycle · Nitrogen fixation · Trophic enrichment Resale or republication not permitted without written consent of the publisherMar Ecol Prog SerMarine plankton have been found to feed opportunistically in unstructured food webs, with particle size as the most important factor (Isaacs 1972, 1973, Platt & Denman 1977, Sheldon et al. 1977, Platt 1985. Size fractions of plankton are therefore likely to be representative of trophic groups in the marine plankton community. The present study investigated size-dependent changes and an annual cycle of δ 13 C and δ 15 N in sizefractionated Baltic plankton, ranging from <1 µm to the jellyfish Aurelia aurita (40 mm)....

Differential δ13C and δ15N signatures among scallop tissues: implications for ecology and physiology

“…In the literature, δ 13 C isotopic increases are reported to occur synchronously with spring phytoplankton blooms (Canuel et al, 1995, Rolff, 2000. Here, the high variability of primary production (Chl a, fig.1) and seston (δ 13 C and δ 15 N values) was not associated with similar variations of the two stable isotopes in the tissues.…”

“…Since the early work of Stephenson et al (1984), several authors have explored seasonal stable isotopic variation in marine organisms (Goering et al, 1990;Riera et al, 1996, Kang et al, 1999Rolff, 2000). Peterson and Fry (1987) and Peterson (1999) emphasized the limitations of isotopic measures, and the need to do both temporal and spatial sampling to fully understand the observed variations.…”

Seasonality of stable carbon and nitrogen isotopes within the pelagic food web of Taihu Lake

“…The characteristic seasonal cycle has been reported in several studies. Rolff (2000) reported a bimodal distribution of δ 13 C, with a spring and autumn maximum enrichment for size-fractionated plankton in the Baltic Sea. Xu et al (2005) found enrichment of δ 13 C in seston and lake anchovy with the progression of the summer bloom.…”

Tracing terrestrial organic matter by δ34S and δ13C signatures in a subarctic estuary

“…Phytoplankton as the marine end member for d 13 C DOC in the open Baltic Sea water column is known to use riverine DIC for production and, thus, areas more influenced by riverine DIC show lighter d 13 C values in phytoplankton. Rolff and Elmgren (2000) measured the d 13 C signature of phytoplankton in the Bothnian Bay between -25% and -27%, in the Bothnian Sea between -22% and -24%, and in the Baltic proper between -19% and -22% (Rolff and Elmgren 2000). The d 13 C signature of phytoplankton is also known to vary Fig.…”

“…The mean difference between the measured and the calculated ratios was 13.5%. seasonally because of varying riverine DIC discharges (Chanton and Lewis 1999;Rolff 2000).…”