Heinz-Dieter Franke

Long-term changes in the macrozoobenthos around the rocky island of Helgoland (German Bight, North Sea)

The paper briefly summarizes what is known about long-term changes (facts, causes, consequences) in the macrozoobenthos of intertidal and subtidal hard-bottom communities around the island of Helgoland (German Bight, North Sea). There is increasing observational evidence that these communities (spectrum and abundances of species) are changing on a long-term temporal scale. The reasons are diverse and mainly anthropogenic. A shift in North Sea climate towards more oceanic conditions may be among the most important factors driving the recent changes in species spectrum. Many of the species which have been recorded as new to the Helgoland area during the past decade are southern (oceanic) species which may be considered as indicators of a warming trend.

The recent arrival of the oceanic isopod Idotea metallica Bosc off Helgoland (German Bight, North Sea): an indication of a warming trend in the North Sea?

In 1988 a long-term study was started of the isopod fauna associated with surface drift material off Helgoland (German Bight, North Sea). In the summer of 1994 specimens ofIdotea metallica Bosc were recorded for the first time. There is no evidence that this species has ever been present in the German Bight before. The samples contained males, both gravid and non-gravid females, and juveniles, indicating that the species reproduced successfully in the Helgoland region. Interbreeding of specimens from Helgoland and the western Mediterranean produced fertile off-spring. As a neustonic species,I. metallica shows a high natural capacity for dispersal. It thus seems unlikely that the arrival of the species in the North Sea resulted from an accidental introduction by man. We are probably witnessing an extension of the species’ geographical range by natural means of dispersal, as a response to recent changes in the ecological conditions of the German Bight. Temperature data measured by the Biologische Anstalt Helgoland since 1962 show that the last decade (except 1996) was characterized by unusually mild winters. Following the severe winter of 1996,I. metallica was again absent from the Helgoland region. After the subsequent mild winters (1997 and 1998), however, the species reappeared in the summer of 1998 with higher numbers than ever before. This suggests that the observed phenomena are closely connected with the recent temperature anomalies.I. metallica can be regarded as a potential immigrant to a warmer North Sea, and may be useful as a sensitive indicator of the predicted long-term warming trend.

On the current and possible future status of the neustonic isopod Idotea metallica Bosc in the North Sea: a laboratory study

In the 1990s, a period of extraordinarily mild winter temperatures in the German Bight (North Sea), the oceanic, neustonic isopod Idotea metallica Bosc was found for the first time off the island of Helgoland. The species was recorded in subsequent summer periods, reproducing successfully in the area, while its whereabouts in winter remained uncertain. The question arises whether a continuing increase in mean winter water temperature, as predicted in most scenarios on future climate, might enable I. metallica to overwinter in the North Sea and to become a permanent resident there. Experiments on laboratory-cultured I. metallica were performed for the first time. Population dynamics was studied at different temperatures in microcosms. Furthermore, the temperature effects on reproductive output, mortality and duration of embryonic development were studied in individually reared animals. The results suggest that at current temperature conditions (mean winter water temperature of about 58C) I. metallica is unable to overwinter in the German Bight, and that even an increase to about 88C probably would not change this situation. The recently observed summer populations of the species in the German Bight obviously originate from individuals introduced each year anew by water currents from the Atlantic Ocean. Nevertheless, I. metallica may be useful as a sensitive indicator: in a warming North Sea the species may become a regular summer immigrant, developing more and more conspicuous populations in this area as the annual period with water temperature above the critical level (138C) for reproduction extends. q 2001 Elsevier Science B.V. All rights reserved.

A supplement to the amphipod (Crustacea) species inventory of Helgoland (German Bight, North Sea): indication of rapid recent change

The surroundings of the rocky island of Helgoland (German Bight, south-western North Sea) are one of the best-studied sites in European seas with species occurrence data available for nearly 150 years. As the area is strongly affected by global change (e.g. increase in mean sea surface temperature at Helgoland by 1.678C since 1962), ecosystem structure and function are expected to change more than those of average marine systems. The paper presents a supplement to the local amphipod species inventory (5 nautical miles around the island). At least seven species of this ecologically important taxon seem to have newly established themselves at Helgoland since the late 1980s: Ampelisca typica, Amphilochus brunneus, Caprella mutica, Cheirocratus assimilis, Monocorophium acherusicum, Orchestia mediterranea and Orchomenella crenata. Most of them are not only new for the Helgoland area, but also for the German Bight; and two species (Amphilochus brunneus and Orchomenella crenata) are even new to the North Sea as a whole. Out of the seven new species, one (Caprella mutica) is a neozoon from the north-western Pacific. The other six species show clear warm water affinities (oceanic–Lusitanian species) which suggest a recent range expansion in the context of climate warming. The establishment of an increasing number of species formerly probably absent from the area does not seem to be compensated by losses of species, so that local species diversity is expected to increase.

Habitat-related differences in the responses to oxygen deficiencies in Idotea baltica and Idotea emarginata (Isopoda, Crustacea)

Abstract Two apparently ecologically similar sublittoral isopod species, Idotea baltica and Idotea emarginata , were studied with respect to their abilities to cope with deficiencies in environmental oxygen concentration. In a first series of experiments, respiration rates of both species were measured, at different temperatures (5, 10, 15°C), as a function of oxygen partial pressure. Whereas I. baltica showed the characteristics of an oxyconformer, I. emarginata regulates oxygen consumption from normoxia down to distinct hypoxic conditions. In a second series of experiments, anoxia survival times (LD 50 ) were determined for different types of individuals of both species (mancas, juveniles, adult females, adult males). In both species, survival times increased with developmental stage. Adult males were more resistant to anoxia than adult females. In all types of individuals, the LD 50 values of I. emarginata were significantly higher than those of I. baltica . The interspecific differences are clearly adaptive, correlating with respective habitat differences. Idotea baltica is associated with seaweed drifting at the water surface where oxygen is in the normoxic range. In contrast, I. emarginata lives among accumulations of macroalgal debris on the seabed where hypoxia is common. The different abilities of the species to sustain periods of hypoxia may contribute to the maintenance of habitat segregation.

Brittle stars (Echinodermata: Ophiuroidea) in the German Bight (North Sea) species diversity during the past 130 years

In this study, an overview of the current species composition of ophiuroids off Helgoland, German Bight (North Sea) is given. In addition, abundance and distribution of ophiuroids on different types of soft bottom sediments taken by van Veen grab samples around Helgoland was recorded. Literature was reviewed in order to outline the diversity of ophiuroid species over the past 130 years in the inner German Bight. In historical literature, quantitative references often apply to verbal descriptions and thus make comparisons to modern data and assessment of possible changes in abundance rather subjective.In total, six ophiuroid species were identified off Helgoland: Acrocnida brachiata (Montagu, 1804), Amphiura filiformis (M?ller, 1776), Amphipholis squamata (Delle Chiaje, 1829), Ophiothrix fragilis (Abildgaard, 1789), Ophiura albida Forbes, 1839 and Ophiura ophiura (Linnaeus, 1758). The species found in this study had also been reported by previous investigators and are regarded as ?common? species in the German Bight. Occasional findings in this area refer to Amphiura chiajei Forbes, 1843, Ophiopholis aculeata (Linnaeus, 1767), Ophiocten affinis (L?tken, 1858) and Ophiura sarsii L?tken, 1858, which are regarded here as ?rare? species. These findings, however, do not indicate sustainable changes in the species diversity over time.Apart from A. brachiata, a newcomer in the 1970s, findings of the common species mentioned above can be dated back to 1875. Therefore, a fairly stable composition of brittle stars is represented in the inner German Bight during the past 130 years.

The influence of intrinsic and extrinsic factors on developmental parameters and their relationships in the marine isopod Idotea linearis (Crustacea)

Developmental and reproductive parameters and their relationships were studied in the marine isopod Idotea linearis. We hypothesized that (1) the temporal patterns of molting and growth undergo complex and sex-specific changes with age as well as with the onset of sexual maturation, and that (2) sexual maturation (and dependent parameters) is controlled by the photoperiod. Both males and females were singly cultured in the laboratory at two alternative photoperiods (constant long and short days, respectively) from hatching until death. Males molted and grew throughout their life, showing a steady increase in stage duration and body size with each molt. Females, in contrast, showed much more complex modifications in molt chronology due to reproductive demands. There was some variability in the stage number, when females reached maturity. Reaching maturity early in the succession of molts was associated with smaller body size at maturity, smaller size of broods, but higher average number of broods per lifetime. Post-puberty molts in females occurred without further growth, and successive broods did not differ in size. The photoperiod strongly affected sexual maturation (and thus in turn molting and growth patterns) in females, while males remained completely unaffected by the photo regime.