Horst Weikert

Evidence of episodic changes in deep-sea mesozooplankton abundance and composition in the Levantine Sea (Eastern Mediterranean)

Mesozooplankton samples from the 4270-m deep Ierapetra Basin in the oligotrophic Levantine Sea have revealed a strong temporal variability in abundance and composition hitherto unknown for the deep sea pelagic zone. The phenomenon reflected by a survey in June 1993 as compared to January 1987 is assessed by (i) the increase of the mesozooplankton standing crop throughout the water column, (ii) the absence of a decline in mesozooplankton abundance with increasing depth below 1000 m, (iii) the outstanding dominance of two interzonal calanoid copepods at bathypelagic and abyssopelagic depths and (iv) a substantial faunal change due to the codominance of Calanus helgolandicus, which was found for the first time in the area. The underlying hydrographical factors are addressed in the light of the Eastern Mediterranean Transient (EMT). Apart from a possible long-term faunal change, the EMT may exaggerate episodic plankton blooms and surface-abyssopelagic coupling in space and time.

Copepod carcasses in the upwelling region south of cap blanc, N.W. Africa

High numbers of copepod carcasses were found in zooplankton samples taken in the upwelling system off N.W. Africa. The validity of the measurements is discussed. It is suggested that rapid changes in the environment, caused by the pulsation of violent mesoscale upwelling at the shelf edge, result in an increased mortality in zooplankton. Considerations are made on the role of copepod detritus in the upwelling region.

The structure and carbon demand of the bathyal benthic boundary layer community: a comparison of two oceanic locations in the NE-Atlantic

Abstract Estimates of standing stocks were used together with metabolic rates from literature to compare the structure and the respiratory carbon demand of the benthopelagic fauna and epibenthic megafauna at two oceanic locations in the northeast Atlantic. The total standing stock of the benthopelagic fauna and epibentic megafauna (in the following referred to as benthopelagos sensu latu) in the Iceland Basin (59°N/20°W) was 5 times higher than at the BIOTRANS site (West European Basin, 47°N/20°W). While fish were the predominating group at the northern location, followed by epibenthic megafauna, most of the biomass at the southern station could be attributed to epibenthic megafauna whereas fish were even surpassed by zooplankton.The overall respiratory carbon demand of the benthopelagos s.l. in the Iceland Basin was about 50% higher than at the BIOTRANS site. In both areas, a large fraction of the carbon was respired by the megafauna, accounting for 46 % of the total respiratory carbon demand in the Iceland Basin and 86% at the BIOTRANS site. Important consumers in the Iceland Basin were also zooplankton (27%) and fish (26%), whereas at the BIOTRANS site only zooplankton had a significant share (12%) besides megafauna. Compared to the carbon fluxes into the BBL, the faunal groups (without bacteria) remineralize ca. 10–20% of the sedimenting POC. If near-bottom pelagic bacteria are included, the fraction of the POC import flux remineralized by the benthopelagic community amounts to at least 20–50%. That means, the carbon flux as measured in sediment traps is not sufficient to fuel both the benthopelagic community including the megafauna plus the sediment community.

Transfer of organic matter in the deep Arabian Sea zooplankton community : insights from δ15N analysis

Abstract Zooplankton samples were obtained in the central Arabian Sea using a 1 m 2 -MOCNESS with 333 μm mesh aperture to investigate the nitrogen stable isotopic composition of different mesozooplankton size classes ( ⩽500 m during April 1997 and February 1998. Samples from the upper 500 m indicated temporal differences between two years, with the lowest value (4.5 ‰ ) for the 1–2 mm size class between 250 and 500 m depth in April 1997 and the highest value (13.5 ‰ ) for the 2–5 mm size class in the same depths range in February 1998. The upper bathypelagic zone (1000–2500 m) showed an increase in δ15N with increasing depth for all size classes. In April 1997, the size-dependent distribution showed higher values in the larger size classes, indicating higher trophic levels as compared to smaller size classes. Such a size-dependent increase was not evident in February 1998. Below 2500 m, the δ15N values were more or less stable with increasing depth, or even decreased as exemplified by the smallest size class ( mm ) . The size-dependent distribution for both investigated periods showed increasing δ15N values with increasing size. These results give an insight in the trophic structure of the zooplankton community in the deep Arabian Sea. Differences between size classes were less than one trophic level in the upper bathypelagic zone (1000–2500 m) and one to two trophic levels in the lower bathypelagic zone (>2500 m ) . The amount of diet needed by the different mesozooplankton size classes to build up the measured biomass is estimated for the deep bathypelagic zone.

Are Rhodophyceae a dietary component for deep-sea holothurians?

Eicosatetraenoic acid (20:4(n-6)), a polyunsaturated fatty acid (PUFA), was found in deep-sea holothurians at a depth of nearly 5000 m in the temperate north-east Atlantic. It has been suggested that deep-sea animals may be able to synthesize this compound de novo . Since this fatty acid is typical of Rhodophyceae, which has previously been found incorporated in gelatinous material captured in the deep sea, an alternative origin of this fatty acid in the deep-sea holothurians is proposed.

The near-bottom plankton community at the Porcupine Abyssal Plain, NE-Atlantic: Structure and vertical distribution

Abstract The study presents results on the composition and vertical distribution of the near-bottom plankton community at an abyssal site in the NE Atlantic. Plankton samples were collected at 1, 15, 50 and 100 m above bottom (mab). Whereas the composition within the upper three layers was very similar, a major shift occurred in the immediate vicinity of the seafloor. Between 100 and 15 mab, the plankton was dominated by Copepoda, making up more than 75% of the total abundance and biomass (without gelatinous organisms). At 1 mab, Copepoda were still abundant, but their share decreased to ca. 50%, while Polychaeta, Malacostraca and Chaetognatha became important groups. Within the Copepoda, the predominance of the genus Metridia (Calanoida) in the upper layers was replaced by the genus Benthomisophria (Misophrioida) at 1 mab. Despite enrichment in organic particles towards the bottom, the total abundance and biomass of plankton did not show marked differences between the four layers investigated. Several hypotheses are discussed which may explain why the presumably higher food concentrations near the deep-sea floor do not lead to increased standing stocks of the plankton community.

Environmental Risks of Mining Metalliferous Muds in the Atlantis II Deep, Red Sea

After two companies were awarded a 30-year license for the exploration and exploitation of metalliferous sediments in the Atlantis II Deep (Red Sea) in 2011, we herewith present conclusions and recommendations derived from an environmental risk assessment, the Metalliferous Sediment Atlantis II Deep (MESEDA) study, conducted in the period 1977–1981. For economic reasons, this program was discontinued before final report delivery and fell dormant for 30 years. The effects of environmental disturbances of the benthic and the near-bottom water layer habitats in and around the mining site deserve further and more modern risk assessments. We examine the relevance of our 1981 recommendations and of subsequent publications to the extended period of resource extraction planned for this century and recommend more up-to-date risk assessment investigations and evaluations.

High temporal variability of zooplankton, but only weak evidence for a near-bottom effect on composition and distribution in the deep Levantine Basin, eastern Mediterranean

ABSTRACT Abundance and composition of the near-bottom zooplankton between 10 and 100 metres above the bottom (mab) were studied in the Levantine Basin, eastern Mediterranean, during four cruises of RV Meteor in June 1993, January 1998, April/May 1999 and October 2001. Copepoda made up 91% of all zooplankton caught. A strong dominance of one single species was observed on all cruises, with Lucicutia longiserrata reaching 50–90% of all Copepoda except in 1993, when Subeucalanus monachus was the most abundant species, with more than 90% of all Copepoda. The year 1993 was also exceptional in terms of total zooplankton abundance, being more than one order of magnitude higher than in the other years. Vertical differences in abundance and composition were small and did not indicate a near-bottom effect or a specialized benthopelagic zooplankton community in the layers sampled.