ALBERT

Description: Donax hanleyanus Philippi, 1847 (Bivalvia: Donacidae) dominates fine to coarse sandy beach communities of the northern Argentinean Atlantic coast. The population biology of this intertidal wedge clam was studied by determining population structure, growth and production at the three locations Santa Teresita, Mar de las Pampas (both from December 2005 to December 2006) and Faro Querandí (from March 2005 to December 2006). Von Bertalanffy growth functions were established from length-frequency distributions using an asymptotic length (L∞) of 44 mm and the growth constants (K) of 0.46 and 0.47 y-1 respectively of Mar de las Pampas and Faro querandí. Compared with growth studies four decades ago, D. hanleyanus today is growing more slowly, but is reaching a higher maximum length. Longevity is estimated to be approximately five years. The present study confirms that the overall growth performance index is habitat-specific, grouping Donacidae into tropical/subtropical, temperate and upwelling species. The intertidal biomass of D. hanleyanus ranged between 0.04 and 1.32 g ash-free dry mass (AFDM) m-2yr-1. Individual production revealed the highest value at 30 mm length (0.16 g AFDM m-2yr-1) and annual production ranged between 0.08 and 0.99 g AFDM m-2yr-1, resulting in renewal rate values (P/) between 0.82 and 2.16. The P/ ratios of D. hanleyanus populations increased with decreasing latitude from temperate to tropical regions. Only at Santa Teresita D. hanleyanus was found living with the sympatric yellow clam Mesodesma mactroides. A significant negative correlation between abundances of both surf clams suggests that abundance peaks of D. hanleyanus are related with population crashes of M. mactroides. Spatial differences in abundance are significantly related to sand texture as confirmed by nonmetrical multidimensional scaling, but not to sea surface temperature. However, the decrease of D. hanleyanus seems to be principally related to human activities.

Description: The Humboldt Current System is a highly productive ecosystem that is subject to the dynamics of the El Niño Southern Oscillation (ENSO). El Niño (EN, the warm phase of ENSO) causes vital changes in surface water temperature, oxygen levels, and salinity conditions, which arc reflected in various responses of coastal pelagic and benthic organisms. For very shallow habitats such as sandy beaches, temperature and salinity are considered the principal parameters changing during strong EN. However, the mechanisms by which these changes effect change on the structure of coastal populations remains largely unknown. The surf clam Donax ohesulus is dominant on large sandy beaches of the Humboldt Current System. Its biogeographical distribution is largely influenced by EN-induced environmental changes. Despite the species' key role in the beach ecosystem, the effects of modified abiotic conditions on the meroplanktonic larval stages and threshold temperatures involved have not yet been investigated. After EN episodes, meroplanktonic larval stages play a crucial role in the medium- and long-term stability of shallow-water species. Thus, this study makes a first attempt to describe the ontogeny of D. ohesulus and examines the effects on development of EN temperature conditions (ENTC) in comparison with normal temperature conditions (NTC). Results indicate that early life history follows a pattern previously described for other donacid bivalves. Development, growth, and mortality of larvae were assessed during a 3-wk in vitro experiment, indicating that larvae reared under ENTC grew and developed faster in comparison with those reared under NTC; mortality was slightly higher under ENTC. During a 2nd experiment, larvae were exposed for 48 h to a distinct range of different salinities (35, 25, 15, and 5 ± 1) at 2 different temperatures (NTC and ENTC). At both temperatures, larvae suffered no mortality at medium and low salinity (35, 25, and 15 ± 1) but showed 100% mortality at very low salinity (5 ± 1) after 16 h at NTC and 32 h at ENTC. Activity of larvae was highest at medium salinity (25 ± 1) and lowest at normal salinity (35 ± 1). The results of this study indicate that early larval stages of D. ohesulus can cope with temperature and salinity changes induced during EN. Only extremely low salinity (5 ± 1) such as that observed close to river mouths may cause high mortality rates in D. ohesulus offspring.

Description: Das Makrobenthos der tieferen Teile der Kieler Bucht wurde im Jahr 1968 auf 24 Bodengreiferstationen im Abstand von zwei Monaten quantitativ erfaßt, um ein Bild der Nahrungsvoraussetzungen für bodenlebende Fische zu erhalten. Im einzelnen wurden der Anteil von Fischnährtieren am Gesamtbenthos, die Menge an Fischnahrung in wichtigen Fischereigebieten sowie jahres- und tageszeitliche Veränderungen des Nahrungsangebots untersucht. Nach einem Vergleich von Biomasse und Produktion verschiedener Benthosarten wird der Versuch gemacht, die Mindestproduktion an Makrobenthos für die Abra alba-Zönose der Kieler Bucht abzuschätzen. A quantitative study of macrobenthos in the deeper parts of Kiel Bay was conducted every two months in 1968 to get a picture of the food available for demersal fish. The share of fish food in the total benthos, the amount of food animals in important trawling localities, and seasonal and diurnal changes in the food offer are particularly discussed. After a comparison of biomass and production of different benthos species, the minimum production of macrobenthos in the Abra alba-coenosis of Kiel Bay is fixed by estimate.

Description: Gelegentlich einer Bearbeitung des Cyprina islandica-Bestands der Kieler Bucht im Jahr 1968 wurde auch der Befall dieser Muschel mit dem kommensalischen Nemertinen Malacobdella grossa ermittelt. Von insgesamt 534 untersuchten Islandmuscheln waren ca. 27% befallen; die Befallsrate ging in den letzten 60 Jahren um mehr als die Hälfte zurück. Größere Muscheln enthielten den Nemertinen wesentlich häufiger als kleine. Regional bestehen Unterschiede, die aber nicht mit der Häufigkeit von Cyprina korrelierbar sind. Juvenile und adulte Nemertinen traten das ganze Jahr hindurch auf; eine Periode intensiver Fortpflanzung war nicht zu erkennen. Eine Schädigung befallener Islandmuscheln durch den Kommensalen ist nicht nachzuweisen. On the occasion of a study of the soft clam (Cyprina islandica L.) stock in Kiel Bay during 1968, the infestation of this bivalve with the entocommensal rhynchocoelan Malacobdella grossa was noted. Of a total of 534 soft clams investigated, about 27% were infested; this is less than half the rate found 60 years ago. Larger bivalves contained the nemertean much more frequently than smaller specimens. There are regional differences, which can, however, not be correlated with the abundance of Cyprina. Juvenile and adult nemerteans occured throughout the year; a period of intensive breeding was not noticed. A negative effect of the commensal on its host cannot be proved.

Description: Tauchuntersuchungen ermöglichten erste quantitative Angaben über das Vorkommen der Bohrmuschelarten Barnea candida und Zirfaea crispata in der Kieler Bucht. Die Muscheln bewohnen in hoher Siedlungsdichte Geschiebemergelflecken zwischen 2 und 13 m Wassertiefe. Ihre Bedeutung für die Auflockerung des Substrats, das dadurch auch von anderen Benthosformen besiedelt werden kann, ist beträchtlich. Diving investigations have provided first quantitative data on the occurence of boring clams (Barnea candida and Zirfaea crispata) in Kiel Bay. In a zone between 2 and 13 metres of water depth, the clams live in patches of till in large numbers. They have a considerable importance for eroding the substratum, which thus can be inhabited by other benthic species as well.

Description: In the scope of our investigations on the basic processes in a soft-bottom macrobenthos community, as a preliminary test, a series of five 1 m2 metal boxes filled with sterilized sediment from the surrounding habitat was exposed in an enclosed area in the western Kiel Bight at a depth of 20 m from September 1972 until July 1975. The establishment and succession of the soft bottom association in the boxes was followed and compared to the association on the surrounding sea floor by monthly to bimonthly sampling by the diving group of SFB 95. While the number of species increases continuously until a final level of about 20 per 0.1 m2 is reached in January 1974, the values of total biomass (the starfish excluded) fluctuate seasonally being high in autumn and early winter and low in March and April. In a late experimental stage in January 1975, before or just in the beginning period of the regression of the bivalve populations, the association in the boxes (80g wet wt./m2) has only reached 50% of the biomass of the assemblage outside where large long-lived and slowly growing bivalve species contribute 75% of macrobenthos weight. The data of total specimen number fluctuate widely due to oxygen deficiency in late 1973 followed by a high colonization activity by opportunistic spionid species immediately after. According to the dominance in specimen numbers of the major taxononomic groups, three phases can be distinguisted: crustacea (mainly Diastylis rathkei) - sedentary polychaetes (spionids, Pectinaria koreni) - bivalves (Abra alba) together with errant polychaetes (Nepthys spp.). In terms of biomass, however, there is an additional phase of echinoderm prevalence (Asterias rubens) during the last five months. The starfish obviously utilize the bivalve production to a high extent: between 94 and 75 % mortality for the three most abundant bivalve species within six months. For the last six months of the experiment, the net production of all bivalve species is calculated as 24g wet wt./m2, i.e. 0.61 g organic carbon of living tissue, most of which is produced by only three species (Abra alba, Cardium fasciatum and Mya truncata: 22g wet wt/m2.) lt is suggested that predators are of eminent importance in controlling succession and production of the new association. The effects of the experimental conditions on the findings is discussed, and a design for extended interdisciplinary in situ experiments to be carried out from 1976 on, based on the experiences of this first test, is presented.