Ingrid Kröncke

Daily Growth Rates in Shells of Arctica islandica: Assessing Sub-seasonal Environmental Controls on a Long-lived Bivalve Mollusk

Abstract Shells of the extremely long-lived bivalve mollusk Arctica islandica (Linnaeus 1767) provide century-long, multi-proxy records of inter-annual environmental variability in middle- to high-latitude marine settings. Reliable interpretation of these climate archives, however, requires exact knowledge of the length and timing of the growing season and which environmental parameters control shell growth rate during the year. Here, intra-annual growth microstructures, δ18O-derived ambient water temperatures, and δ13C from A. islandica shells collected from the southern and central North Sea are studied. Such data were analyzed in conjunction with observational sea-surface temperature and primary productivity data. Arctica islandica produces circadian growth increments in its shell (on average 31.5 μm per day during age four, measured along the outer shell surface), which allow assignment of calendar dates to each shell portion. The growing season of A. islandica in the upper mixed layer of the ocean (here 25 m water depth) is not continuous over an eight-month period as previously suggested. Rather, it is interrupted during spawning between early September and mid-November. In addition, shell production ceases or is strongly retarded due to food scarcity between mid-December and mid-February. Water temperatures derived from oxygen-isotope ratios are in good accord with observed sea-surface temperatures. In specimens at 25 m water depth, abrupt changes in shell δ18O-derived temperature (Tδ18O) were interpreted to represent vertical displacements of the seasonal thermocline. Daily shell growth is controlled by temperature and food availability. Up to 58% of the variation in daily growth rate is explained by these environmental parameters. This study demonstrates that A. islandica can provide seasonal to subseasonal, precisely dated proxies of environmental variables. Such data are of increasing importance for climate models.

Influence of macrofauna long-term natural variability on benthic indices used in ecological quality assessment

An essential prerequisite for the assessment of the ecological quality of marine ecosystems is the understanding of the natural variability and its effect on the performance of quality indices. This study is focused on the long-term natural variability of diversity, biotic and multimetric indices by using long-term macrofauna data of a coastal area in the southern North Sea (1978-2005). The univariate and most biotic and multimetric indices respond significantly on specific natural disturbance events such as cold winters, but the strength of response varied between indices as well as between events. As a result, the ecological quality status can decrease over a range of 3 (out of 5) classification units. The overall ecological quality was good to high, but an increase of indices occurred from the mid 1980s onwards due to changes in the climate regime. This long-term variability has to be considered within ecological quality assessment schemes.

Long-term variability in macrofauna species composition off the island of Norderney (East Frisia, Germany) in relation to changes in climatic and environmental conditions

Macrofauna samples were collected quarterly from 1978 to 1999 in the sublitoral zone off Norderney, one of the East Frisian barrier islands.Abundance, biomass and species numbers of single species or taxonomic groups showed different long-term variability. Different species distribution patterns were found over time, according to different zoogeographical distribution or feeding mode. Temperature seemed to be a major factor in structuring the community. Cold winters in 1978/79 and 1995/96 had severely negative effects on the community. A clear shift in community structure has become obvious since 1988; this occurred in direct connection with an increasing North Atlantic Oscillation Index (NAOI). The mediator between the NAO and the benthos is the sea-surface temperature (SST). Additionally, increasing storm frequency since the late 80s has changed the hydrodynamic in the area and led to a higher resuspension of sediments, which changed the food availability for macro-fauna species, as well as sediment stability. The results are similar to other long-term changes in the North Sea and indicate a North Sea system shift at the end of the 80s, which was forced by climatic factors.

Macrofauna communities of the Dogger Bank (central North Sea) in the late 1990s: spatial distribution, species composition and trophic structure

Macrofauna samples were taken in May 1996–1998 at 28 stations on the Dogger Bank (central North Sea) to describe species composition, spatial distribution and trophic structure of the macrofaunal communities. In general, five communities could be distinguished depending on the influence of different water masses, depth, type of sediment and food availability. Except for those species which feed in the sediment, macrofauna communities did not show any correlation to the measured sediment variables. In fact, macrofauna communities of the Dogger Bank were mainly structured by lateral food supply via frontal systems. This was indicated by the dominance of interface feeders foraging in the benthic boundary layer, which are able to switch from surface deposit to suspension feeding depending on flow conditions. The importance of sandlickers in the shallowest parts of the Dogger Bank hints at an important contribution of benthic primary production to the nutritional and energetic needs of the benthic community.

How predictable is high bivalve recruitment in the Wadden Sea after a severe winter

Higher than average recruitment among bivalves on the intertidal flats of the Wadden Sea was observed many times after severe winters in the period 1940 - 1995. The occurrence of another severe winter in 1995/96 prompted us to test the hypothesis of severe winters leading to universally high bivalve recruitment on a large geographic scale (500 km coastline) in temperate shallow waters. We analysed data sets on bivalve abundance from seven areas in the Dutch, German and Danish Wadden Sea. The longer data sets showed generally higher bivalve recruitment in the 1970Žs and 1980Žs than in the 1990Žs which may be related to the near absence of severe winters since 1987. Considering the period 1988 onwards (the longest possible period in which 1995/96 was the only severe winter), recruitment of Cerastoderma edule was in 1996 - in agreement with our hypothesis - above average at all seven investigated areas. In contrast, recruitment of Macoma balthica and Mya arenaria was for the same period above average only in the southern Wadden Sea (south-west of Jade Bay) but not in the northern Wadden Sea (north of Eiderstedt peninsula). These regional differences may be related to (i) the different topography of the northern Wadden Sea (with barrier islands westwards to the mainland) compared to the southern Wadden Sea (with barrier islands northwards to the mainland) and subsequent differential effects of wind induced currents on bivalve recruitment, (ii) differences in biotic factors such as standing stocks, larval supply or epibenthic predation or (iii) changes in environmental conditions. Our results demonstrate that large-scale comparisons along coasts are an indispensable addition to insights derived from local studies alone.

Macrofauna standing stock of the Dogger Bank. A comparison: III. 1950–54 versus 1985–87. A final summary

The macrofauna communities on the Dogger Bank in April/May 1985–87 were compared with those in April/May 1950–54 (Ursin, 1960). Unpublished data from Birkett on the central and western Dogger Bank from April/May 1952–54 were also used for the comparison. The changes in the communities from the fifties to the eighties were made clear by the increasing species numbers and in an increase or dominance of short-living opportunistic species. In contrast, a decrease in long-living bivalves was found. The total biomass in 1985–87 was 2.5 to 8 times higher than in 1950–54. Similarities between stations were below 20%. Some hypotheses concerning natural and anthropogenic impacts are given to interpret the observed changes.

Decadal changes in macrofauna communities on the Dogger Bank caused by large-scale climate variability

In order to investigate, whether the rise in the North Atlantic Oscillation (NAO) index and the connected change in the hydroclimatic regime of the North Sea since 1988 caused changes in offshore macrobenthic communities, 28 stations on the Dogger Bank (Central North Sea) were revisited in May 1996–98, which were previously sampled in May 1985–87. Compared to the 80s, abundances of southern and interface-feeding species increased in the 90s on top and in the southern parts of the bank, whereas abundances of northern species decreased. Along the northern slope of the Dogger Bank abundances and total number of species, which prefer coarser sediment, increased in the 90s as well as diversity of feeding types and total number of northern species, whereas abundances of species preferring fine sand and interface-feeding species decreased. The results are discussed in relation to changes in temperature, primary production and the current regime of the central as well as the entire North Sea, which in turn are connected to the NAO.

20 years of the German Small-Scale Bottom Trawl Survey (GSBTS): A review

AbstractThe German Small-scale Bottom Trawl Survey (GSBTS) was initiated in 1987 in order to provide complementary investigations to the International Bottom Trawl Survey (IBTS) in the North Sea, using the same methodology but focussing high-intensity sampling on selected survey areas. Over the last 20 years, the initial number of 4 survey areas (10 × 10 nautical miles; “Boxes”) has been increased to 12, which are distributed over the entire North Sea. This paper describes the survey methods of the GSBTS, summarizes the scientific outcome of the first 20 years, and suggests that international fisheries research institutions would join the GSBTS.The major outcomes of the survey include to date:— Documentation changes in the distribution of fish species and in species assemblages (e.g. changes in species richness, shifts in the southern species component).— Geostatistical evaluation of GSBTS data.— Analysis of spatial scale effects: the relevance of GSBTS survey results for interpreting large-scaled abundance and distribution data from the IBTS.— Description of benthic habitats, composition of invertebrate fauna and its variability.— Process studies, especially investigation of predator-prey interactions between fish through analyses of stomach contents.— Characterization of the typical hydrographic conditions in the survey areas and their variability, and description of the nutrient supply.— Observations of seabirds and their feeding habits.— Analysis of the effects of different parameters on catch rates for bottom fish and on the estimates of abundance indices (e.g. vessel and gear effects, towing time, hydrographic conditions, time of day, number of hauls per area). In continuing this interdisciplinary survey with simultaneous sampling of all faunal and environmental compartments and especially in making it an international effort, we see the possibility of contributing data for the implementation of the ecosystems approach to fisheries management. Particularly, the following aspects can be addressed and would further increase the scientific value of the GSBTS:— Combining the survey data with highly resolved data from the commercial fishery to separate the effects of fishing from natural variability.— Further interdisciplinary analyses of the entire data set. Main aspects include benthos-fish-bird-community changes over time and their relation to historic fisheries impacts, and the coupling of biological and physical habitat characterisation.— Collection of accompanying data (phyto-, zoo- and ichthyoplankton data) in order to make the GSBTS a true ecosystem survey in detecting temporal changes in nearly all major levels of the food web.

Invertebrate associations with gastropod shells inhabited by Pagurus­bernhardus (Paguridae) – secondary hard substrate increasing biodiversity­in North Sea soft‐bottom communities

The crustacean Pagurus bernhardus is an abundant hermit crab species in the soft‐bottom benthic communities of the North Sea. The gastropod shells inhabited by the hermit crab were found to provide substrate for a diverse epizoic fauna. In order to describe the epizoan community as well as spatial differences in epizoan communities of the North Sea, hermit crabs were sampled during summer 2001 at five stations in the North Sea. In total, 51 epizoic species with up to 15 species and 427 individuals per crab were found. The most abundant epizoans were obligate associated species such as the polychaete Circeis amoricani paguri, the cnidaria Hydractinia echinata and the crustacean Trypetesa lampas, as well as sessile epizoans also found in hard‐bottom habitats such as balanids Balanus crenatus and Verruca stroemia or free‐living species such as the amphipod Gammaropsis nitida. The epizoic community structure on shells of the stations north of the 50 m contour differs from the southern communities on the Dogger Bank and in the German Bight. The northern stations were dominated by sessile species, whereas a higher proportion of free‐living epizoans occurred at the southern stations. The results are discussed concerning the ecological importance of shells inhabited by hermit crabs as a hard substrate in soft‐bottom habitats.

The cornucopia of chilly winters: Ocean quahog (Arctica islandica L., Mollusca) master chronology reveals bottom water nutrient enrichment during colder winters (North Sea)

We studied variations in annual shell growth rates of ten live-collectedArctica islandica specimens from three localities at the Dogger Bank in the central North Sea. Synchronous growth curves from contemporaneous specimens enabled the construction of a 65-year multi-locality master chronology. Stepwise multiple linear correlation analysis indicated a highly significant (p<0.01) negative relationship between annual shell growth and winter sea surface temperatures (SST) during the period of 1953 to 1995. Up to 27.5% of the variation in the annual shell growth is explained by winter temperature. We explain the higher growrh levels in colder winters by higher food supply. During warmer winters both the stratification and bottom water currents in the study area seem to be stronger and prevent downward-mixing of nutrients as well as settlement of food on the sea floor. During colder winters, however, the stratification weakens and phytoplankton produced in the cold surface layer reaches the sea floor. Long-term changes in shell growth rates are thus directly connected to hydrographic changes in the North Sea, which in turn are relared to fluctuations of the North Atlantic Oscillation (NAO).Our study demonstrates the usefulness ofArctica islandica for long-term, high-resolution environmental reconstructions and contributes to further understanding natural and anthropogenic variability in the environmental history of the North Sea.ZusammenfassungDie vorliegende Studie beschäftigt sich mit Variationen im jährlichen Schalenzuwachs von zehn lebend gefangenenArctica islandica Individuen von drei Lokalitäten auf der Doggerbank in der zentralen Nordsee. Aufgrund synchroner Verläufe der Wachstumskurven zeitgleich lebender Individuen konnten wir eine 65 Jahre umfassende Master-Chronologie erstellen. Schrittweise multiple lineare Korrelation indiziert einen hochsignifikanten (p<0.01) negativen Zusammenhang zwischen jährlichem Schalenzuwachs und Oberflächenwassertemperaturen (SST) während der Wintermonate über den Zeitraum von 1953 bis 1995. Bis zu 27,5% der Variation im jährlichen Zuwachs läßt sich durch Schwankungen der Wintertemperaturen erklären. Wir nehmen an, daß die höheren jährlichen Schalenzuwächse in kälteren Wintern mit höherem Nahrungsangebot zusammenhängen. Während wärmerer Winter scheint die vertikale Schichtung der Wassersäule im Nordosten der Doggerbank stärker zu sein. In Verbindung mit erhöhten Strömungen beeinträchtigt sie das Absinken des Phytoplanktons zum Meeresboden und verhindert ein Durchmischen von nährstoffreichen Oberflächenwassern und tieferen Wassermassen. Über längere Zeiträume sind die Nahrungsverfügbarkeit und damit das Schalenwachstum von Muscheln (hierArctica islandica) direkt an hydroklimatische Veränderungen gekoppelt, die in Verbindung mit der Variabilität der Nordatlantischen Oszillation (NAO) stehen.Unsere Untersuchungen demonstrieren, daß sichArctica islandica für langzeitliche und hochauflösende Umweltrekonstruktionen eignet. Unsere Studie leistet darüber hinaus einen Beitrag zum besseren Verständnis natürlicher und anthropogener Variabilitäten der Umweltgeschichte der Nordsee.