Claudia Kruschel

Sediment movement is a major force in the evolutionary ecology of seagrasses. Sediment movement can cause retreat or expansion of the seagrass bed, and sorting of species according to their tolerances of erosion or deposition. The objectives of this study were 1) to assess the effectiveness of DGPS/videography in demonstrating sediment movement and seagrass bed expansion, 2) to quantify changes in species composition or bed expanse/retreat in response to sediment movement using this method. During 2007, 2008 and 2009, 23 transects in the Novigrad Sea were monitored using a boat-based DGPS videography. Three seagrass species occur in this area: Zostera noltii, Zostera marina and Cymodocea nodosa. They form a continuous mixed meadow from the surface to a depth of 6,6 m on sandy or muddy substrate. Video analysis with sonography gave us information on ground type, ground cover, seagrass density, and depth. Seagrass gain occurred in 2007/2008, the year of net sediment accumulation, whil...

Sažetak: In the Mediterranean, the seagrass Posidonia oceanica (L.) Delile is considered highly threatened and therefore holds habitat priority status under Natura 2000. Because the extent and health of Posidonia in the Croatian Adriatic is currently unknown, we ...

Predation of fish assemblages in seagrass meadows was examined in the field and in tank experiments. Lure trolling indicated that (1) total abundance of fish was higher on bare sediment where small fish (<5 cm), including juveniles, predominated; (2) abundance was lowest in seagrass where large fish (>15 cm) predominated; (3) large ambush predators, primarily the grass goby and European eel, were almost completely restricted to seagrass; (4) the predation mode in seagrass was almost entirely ambushing or stalk-attacking, while the predation mode on bare sediment was almost entirely chase-attacking; (5) ambush predation was far more successful than chase-attack predation; and (6) overall predation risk was approximately three times higher in seagrass. Tank experiments showed that piscivory success of the grass goby was higher than that of the most common chase-attacker, the black goby, and the presence or absence of artificial seagrass, regardless of density, had no significant effect on predation success of either species. Guts of the grass goby contained food items of a wider size range that averaged twice the size of those of the black goby. Our results confirm our prediction that the risk of predation, especially of small/juvenile fish, is higher in seagrass meadows than at adjacent bare substrate, and this risk differential is explained by the presence of larger, more efficient ambush predators restricted to seagrass, and the scarcity of large chase-attack predators in the Novigrad Sea.

Within the scope of a seagrass monitoring program in the Novigrad Sea, Central Croatian Adriatic, we predicted that the annual variability in coverage of seagrasses (Zostera marina, Zostera noltii, and Cymodocea nodosa) can be partially explained by the annual variability in sediment translocation. From 23 fixed DGPS-referenced monitoring video transects followed over three years (June 2007–2009), we calculated annual (i) changes in interior bed seagrass coverage, (ii) gain in seagrass at the lower edge of the bed and seagrass bed expansion, and (iii) accumulation of sediment, its depth dependence, and the associated changes in transect slope. We found that in 2007 to 2008, the year with net sediment accumulation, seagrass coverage increased and the bed expanded. In both years seagrass cover within the seagrass bed increased with increasing sediment accumulation, while seagrass bed expansion was highest under intermediate sedimentation rates. Boat-based videographic monitoring can document both natural sediment movement along the depth gradient, and species-specific responses necessary for informed management of submerged aquatic vegetation in the Adriatic Sea.► Boat-based underwater videography is a rapid and inexpensive method of monitoring small changes in seagrass cover. ► This method was applied to a mixed-species sward of seagrasses in a Croatian Adriatic lagoon. ► Seagrass cover increased over a two year period coincident with a net sediment influx. ► Sediments can increase seagrass habitat by filling in areas too deep for sufficient light penetration. ► Sediment movement may also eliminate species relatively intolerant of burial (such as Zostera marina or Zostera noltii) and favor those more tolerant (such as Cymodocea nodosa).

The vertical gliding motility patterns of Oscillatoria cf. laetevirens and Spirulina cf. subsalsa from hypersaline ponds near Guerrero Negro, Mexico, were monitored under natural solar irradiance. Each of these two filamentous cyanobacteria (and together) forms a distinct band within soft microbial mats. Upward and downward migration in excised mats was measured in response to different intensities of the full solar spectrum and of selected wavelengths, using various filters and screens. Positions of the cyanobacteria were quantified at the beginning and end of the treatments, using microscopic examination of minicores. In addition, photosynthetic rates of these cyanobacteria, using freshly collected cell material, were measured by 14C incorporation under different intensities and spectral regions of solar irradiance. Upward migration to or near the surface by either Spirulina or Oscillatoria occurred under low visible light (20–90 W m−2), green light (∼250 W m−2), red light (∼470 W m−2) and in complete darkness. It was prevented by intensities of UV-A above ∼1.5 W m−2 and by broad visible light above ∼100 W m−2. Incident UV-B intensities as low as 0.1 W m−2 may also have slowed movement upward. Downward migration of Spirulina cf. subsalsa was promoted by high UV or visible radiation. With a larger data set for Oscillatoria cf. laetivirens, it was apparent that downward migration occurred in response to visible light over ∼400 W m−2 and to relatively high levels of UV-A alone (>∼10 W m−2). If forced experimentally to remain on the surface during periods of high solar irradiance, both cyanobacterial species suffered photo and UV inhibition of photosynthesis, with UV-B and UV-A being particularly effective. Attenuation measurements within the mat showed that UV-A and UV-B wavelengths can penetrate deeply enough into mats to potentially act as positioning cues for these motile cyanobacteria, and as a consequence allow them to take refuge from higher intensities of damaging radiation.