Growth of Mytilus edulis in net bags transferred to different localities in a eutrophicated Danish fjord

The growth rate has been measured in Mytilus edulis transferred in net bags to seven localities in the brackish Danish fjord, Limfjorden, in which certain areas are heavily eutrophicated. The increase in shell length, shell weight and flesh body weight was measured after growth periods of 14–18 days. The increase in, e.g., flesh dry weight ranged from twice the start weight to a four-fold increase of the start weight. The net growth efficiencies were estimated to be between 54 and 73%. Algal concentration was in no case the limiting growth factor, but low growth rates were observed in areas with seasonal oxygen depletion and release of toxic H₂S from the sediments. It is suggested that measurements of actual growth in M. edulis can be a useful technique in the study of biological effects in marine recipients.     

Surface waves in isotropic,laterally inhomogeneous media

Summary Two different viewpoints of the phase velocities of the elastic surface waves in isotropic, laterally inhomogeneous media have led to inconsistent results. Arguments in terms of surface wave modes give the conclusion that the phase velocity is independent of the propagation direction, while the outcome of calculations based on a constructive interference of body waves in a surface layer is that the phase velocity is dependent on the propagation direction. Both arguments are summarized and an error in the calculations giving dependence is pointed out. The calculations and observations of surface wave amplitude changes in laterally inhomogeneous media are also summarized.     

Filter-feeding ascidians (Ciona intestinalis) in a shallow cove: Implications of hydrodynamics for grazing impact

The grazing impact by a dense population of filter-feeding ascidians Ciona intestinalis on horizontally flowing water (driven by density circulation) in a shallow cove (Kertinge Nor, Denmark) has been described and quantified by means of a simple one-dimensional numerical model. The agreement between observations and modelled predictions was satisfactory. The applied numerical model has the following analytical solution in the idealized case: Cx = C₀e^−(fx/Y₂), where C_x = algal concentration at a downstream distance x, C₀ = initial concentration, f = F/v_c; F = area specific population filtration rate; v_c = current velocity; Y₂ = depth of mixed layer below halocline. The numerical model quantifies the actual grazing impact while the analytical model illustrates the governing physics in well-known terms. To describe situations with no current (i.e. stagnant water), we performed simulation studies in the laboratory and measured vertical profiles of algal cells over filter-feeding C. intestinalis. The results showed that phytoplankton became reduced in a near-bottom water layer of 20–30 cm thickness. Such water layers may develop in stagnant water (calm days and no advective currents), thus uncoupling the pelagic food and the filter feeders which within a short time will experience extremely meagre food conditions.     

Canopy cover effects on local soil water dynamics in a tropical agroforestry system: Evaporation drives soil water isotopic enrichment

Despite the widely held assumption that trees negatively affect the local water budget in densely planted tree plantations, we still lack a clear understanding of the underlying processes by which canopy cover influences local soil water dynamics in more open, humid tropical ecosystems. In this study, we propose a new conceptual model that uses a combination of stable isotope and soil moisture measurements throughout the soil profile to assess potential mechanisms by which evaporation (of surface soil water and of canopy‐intercepted rainfall) affects the relationship between surface soil water isotopic enrichment (lc‐excess) and soil water content. Our conceptual model was derived from soil water data collected under deciduous and evergreen plants in a shade grown coffee agroforestry system in Costa Rica. Reduced soil moisture under shade trees during the “drier” season, coinciding when these trees were defoliated, was largely the result of increase soil water evaporation as indicated by the positive relationship between soil water content and lc‐excess of surface soil water. In contrast, the evergreen coffee shrubs had a higher leaf area index during the “drier” season, leading to enhanced rainfall interception and a negative relationship between lc‐excess and soil water content. During the wet season, there was no clear relationship between soil water content and between lc‐excess of surface soil water. Greater surface soil water under coffee during the dry season may, in part, explain greater preferential flow under coffee compared with under trees in conditions of low rainfall intensities. However, with increasing rainfall intensities during the wet season, there was no obvious difference in preferential flow between the two canopy covers. Results from this study indicate that our new conceptual model can be used to help disentangling the relative influence of canopy cover on local soil water isotopic composition and dynamics, yet also stresses the need for additional measurements to better resolve the underlying processes by which canopy structure influences local water dynamics.     

Possible mode conversion between Love and Rayleigh waves at a continental margin

Summary. Mode conversion at a continental margin between normal modes of surface waves is investigated by theoretical calculations for oblique incidence for periods longer than 15 s. It is suggested that significant conversion takes place between the various modes of Love waves in the period interval between 15 and 40 s, while there is negligible mode conversion for longer periods. The largest mode conversion involves the lowest modes. In addition the calculations have revealed a small but significant conversion between Love and Rayleigh fundamental modes around 20-s period. Reflections of Love waves are found to be significant only for the continental fundamental mode.     

Seismicity and observations of Lg wave attenuation in Greenland

Earthquakes of local magnitudes between and have occurred in Greenland in the last few decades. Most of the earthquakes are located near the coasts. Some of the earthquake zones are connected to known tectonic features in and near Greenland.The magnitude determinations are based on Lg wave amplitudes and are calibrated using the m_b determinations of the International Seismological Centre. The attenuation of the Lg waves differs from region to region. Along some paths crossing younger geologic features the attenuation is large compared to that in the paths of the old crystalline shield rocks. The Lg wave propagation is influenced by the Greenland ice cap. All the long paths go through the interior of Greenland with its 2–3 km ice cap thickness and they exhibit relatively large Lg wave attenuation.