Relationship between growth parameters of the amphipod Gammarus zaddachi (Sexton 1912) and the permeable body surface area determined by the acid-base titration method

More than 10000 specimens of the amphipod Gammarus zaddachiwere collected from the River Hunte (Huntebrück, Germany) in January, April, July and September 1999 and Jan 2000 to analyse relationships between body-length distributions, growth and the permeable body surface area (from which ions can enter the cell) as indicated by the acid–base titration method. Thus, body wet weights (BWWs), dry weights (BDWs), length (BL), surface area and specific surface area were measured. Mean BWWs were 56.0 mg, BDWs 11.1 mg and BLs 16.4 mm. The relationship between BDW and BL could be successfully described by the power-function: BDW=0.0029×BL ^2.88, probably indicating that the body volume of the test animals can be approximately expressed by an `equal-effectiveness ball'. The body-length distributions were analysed using two combined normal-distribution equations, suggesting that the population of G. zaddachi at Huntebrück was composed of two generations. A sigmoid logistic equation was applied to estimate the body length-based growth. It can be inferred that the reproduction must have occurred around October to December 1998 and that the size of juveniles in the brood pouch was 0.1 mm. The permeable body surface area (ABSA) of G. zaddachi was measured by applying the acid–base titration method, with an average measurement accuracy of 13.7%. ABSA ranged from 17.1 to 236.9 cm² animal^–1 in a variety of the body lengths studied. The specific surface areas (SSA), equivalent to the ratio of permeable body surface area to body volume, decreased from 14.2 to 8.5 cm² mg^–1 (dry wt) as BL increased from 8.1 to 18.6 mm and then remained nearly constant up to 24.1 mm, indicating a single exponential decay relationship between SSA and BL. Mean body length (BL _mean) of G. zaddachi collected simultaneously at Huntebrück increased seasonally from 0.1 mm in autumn–winter to 17.6 in next winter–spring, resulting in an increase of ABSA but a decrease of SSA. Our present results provide a sound and necessary basis for a quantitative study not only on the age-dependent biological/physiological conditions but also on size-dependent bioconcentration of metal by gammaridean amphipods.     

Effects of survey scale and water depth on the assessment of spatial distribution patterns of selected fish in the northern North Sea showing different levels of aggregation

Geostatistics was employed to investigate spatial structuring of herring, cod, dab, haddock and whiting at different spatial scales in the northern North Sea. Additionally, a structural analysis of the maximum water depth was carried out to assess habitat associations of fish. Linear, spherical, exponential and Gaussian models were fitted to the semivariograms, showing clear spatial autocorrelations. At the smaller scale, spatial structuring was weak for haddock, herring and dab, increasing at the greater spatial scale, with the exception of whiting. Mean catch rates, estimated classically and geostatistically, were in good agreement. Corresponding variances were clearly reduced at both spatial scales, when accounting for spatial distribution of the fish. At the greater survey scale a high level of habitat association was detected for haddock and whiting, while a poor habitat association was found for cod, dab and herring. The smaller scale seems to be the threshold at which spatial structuring of “cpue” could have marked influence on estimation error. Thus, survey scale is important when analysing spatial patterns and estimating mean biomass indices, and a sound analysis of relations in spatial structuring of fish and habitat conditions is essential to derive more precise estimates.     

Toxicokinetic models as predictive tools in biomonitoring metals in zooplankton: a synthesis of ideas

The objective of the present study was to evaluate previously reported compartment models as a predictive tool, in particular, to investigate the potential of nonlinear predictions in case of saturation of the uptake process. Examples supporting this idea are presented for the amphipods Gammarus oceanicus from northern Norway and Themisto abyssorum from the Greenland Sea as well as for the copepod Calanoides acutus from the Weddell Sea. A further objective was to investigate verification of model parameters with independent experimental observations, aided by the statistical analysis of the results. This can be regarded as a first step toward a meta-analysis. While modeling results obtained for T. abyssorum in 1993 underestimated observed uptake of Pb and Cu in independent experiments (1999) in the same species, a good agreement was encountered between models obtained for T. abyssorum in 1993 and experimental data obtained for Themisto libellula (1999). Furthermore, a fairly good agreement was found between modeling results obtained for Pb in Calanus hyperboreus from the Greenland Sea (1993) and experimental data obtained for C. acutus (1999). Therefore, the biological species cannot be regarded as a “natural” unit in biomonitoring studies without further evaluation (viz. calibration as biomonitor), while even samples of animals belonging to different species may serve as such a unit, allowing for a comparison of bioaccumulation on large temporal and spatial scales. Finally, the concept of the sensitivity of zooplankton organisms for biomonitoring was elaborated in detail. These procedures involve analytical and experimental variability and model simulations based on previously derived parameters of compartment models. The minimal increments in exposure concentrations estimated in this way were 0.4–1.2 μg Pb l⁻¹ for T. abyssorum, T. libellula, C. hyperboreus, and C. acutus. Corresponding results for Cu varied to a greater extent: 4–9 μg l⁻¹ for the amphipods and 1–5 μg l⁻¹ for the copepods. A correction of these values is proposed, taking into account the ratios of experimental (kinetic) and field bioconcentration factors; this yields exposure concentrations close to those reported for natural seawater. Guest editors: U. M. Azeiteiro, I. Jenkinson & M. J. Pereira Plankton Studies     

Trace Metals in Mesozooplankton of the North Sea: Concentrations in Different Taxa and Preliminary Results on Bioaccumulation in Copepod Collectives (Calanus finmarchicus/C. helgolandicus)

Trace metals (Cd, Cu, Zp, Pb, Ni) were determined in different zooplankton taxa to assess their role in the biogeochemical cycle in the North Sea. Results of semi-static bioaccumulation experiments on board ship using collectives of Calanus finmarchicus/C. helgolandicus were in agreement with a net accumulation strategy, thus meeting an inevitable precondition for their utilization as biomonitors. Cd, Cu and Zn concentrations in field samples of zooplankton displayed a pronounced variability between taxa, with Cd ranging from 0.13 mg kg-1 (d.w.) in fish larvae to 5I mg kg-1 in hyperiid amphipods. Almost no information is available about metal requirements of copepods. However, most of our and worldwide reported Cu concentrations in calanoid copepods (6-22 mg kg⁻¹) coincided with estimates of enzymatic requirements of decapod crustaceans. Cd levels in Calanus collectives increased significantly from 0.68 mg kg-1 in the German Bight to 11 mg kg⁻¹ at some stations around Scotland above 59° N, in line with literature data reported for benthic decapod crustaceans.     

Size-dependent bioaccumulation of metals in the amphipod Gammarus zaddachi (Sexton 1912) from the River Hunte (Germany) and its relationship to the permeable body surface area

The present study aims to evaluate and verify toxicokinetic models for the bioaccumulation of Cd, Pb, Cu and Zn in the gammaridean amphipod Gammarus zaddachi (Sexton 1912) from the River Hunte (Germany). The bioaccumulation experiment was performed in a static system, taking into account the effect of body size on bioaccumulation and the relationship between toxicokinetic model parameters and the permeable body surface area of gammarids. A modified two-compartment model was employed, which was not limited to changes in both biomass of gammarids in the experiments and metal exposure concentrations; the result was a significant model fit. The parameters k ₁ and BCF decreased with increasing body length (BL) of G. zaddachi, while no such trend was observed for k ₂ among BLs ranging from 8.1 to 24.1 mm. The nonlinear relationship was successfully quantified using an inverse-sigmoid logistic model as a basis for subsequent adjustment of the toxicokinetic uptake models. Moreover, k ₁ and BCF increased with increasing specific surface area (SSA, i.e., the ratio of permeable body surface area to body volume) of gammarids. This relationship was successfully quantified using an extended Langmuir equation, which was derived by combining the inversely sigmoid logistic relationship between k ₁, BCF_Exp and BL with a single-exponential-decay relationship between SSA and BL reported previously, implying that the size-dependent bioconcentration was dominated by the SSA-related uptake. Thus, with increasing SSA, k₁ and BCF at first increased sharply, for smaller SSAs with larger BLs, but then approached saturation for larger SSAs with smaller BLs. In addition, field-to-experimental BCF ratios (BCF_Field/BCF_Exp) were determined, yielding values around 1 (indicating equality of the two BCFs) for Pb and Cd for smaller amphipods, but much higher values for larger sizes. The BCF ratios for Cu and Zn were much larger than 1 for both smaller and larger sizes. However, when seasonal changes in BL distribution of gammarids were considered, no significant differences were observed between annual ranges of BCF_Field and BCF_Exp for Pb and Cd. Considering the seasonal changes in BL distribution as well as the Cu and Zn metabolic requirements, no significant difference was observed for Cu, but still a significant one for Zn. From an ecotoxicological perspective we suggest that in the verification of toxicokinetic models not only field-to-experimental BCF ratios should be taken into account, but also several ecological factors such as the size distribution of the animal populations under study as well as, if applicable, metabolic requirements for essential elements.