Ole Geertz-Hansen

Depth Colonization of Eelgrass (Zostera marina) and Macroalgae as Determined by Water Transparency in Danish Coastal Waters

We present a comparative analysis of lower depth limits for growth of eelgrass, large brown algae and other macroalgae measured by SCUBA-diving along 162 transects in 27 Danish fjords and coastal waters, coupled to 1,400 data series of water chemistry (especially nitrogen) and Secchi depth transparency collected between March and October. Danish coastal waters are heavily eutrophied and characterized by high particle concentrations, turbid water and lack of macrophyte growth in deep water. Median values are 3.6 m for Secchi depth and median lower-depth limits are 4.0 m for eelgrass, 5.3 m for brown algae and 5.0 m for other macroalgae. Depth limits for growth of eelgrass and macroalgae increase linearly with transparency in the coastal waters. The relationships are highly significant (p<10−6) and transparency accounts for about 60% of the variability of depth limits. Eelgrass extends approximately to half the maximum depth of macroalgae, presumably because of greater respiratory costs to maintain the below-ground rhizomes and roots of eelgrass, which often constitutes half the plant weight. As a reflection of the importance of total nitrogen (TN) in controlling phytoplankton biomass and thus Secchi depth in coastal marine waters, we found that TN could explain 48–73% of the variation in depth limits of eelgrass and macroalgae, according to a multiplicative model (Y=aXb). As with Secchi depth, the relationship to eelgrass showed a lower slope, reflecting the higher respiratory costs of eelgrass. The models show great sensitivity and a profound quantitative response with proportional effects on Secchi depth and depth limits when total-N concentrations are reduced.

Growth and grazing control of abundance of the marine macroalga, Ulva lactuca L. in a eutrophic Danish estuary

Abstract The marine macroalga, Ulva lactuca L. in its free-floating form was abundant throughout the year in the eutrophic inner parts of a Danish estuary and scarce in the outer parts. The growth rate of Ulva was not significantly different between the inner and outer parts. Grazing by invertebrates was negligible in the inner part and allowed biomass accumulation, whereas grazing pressure in the outer part matched the growth rate during summer and exceeded it by more than two-fold during autumn. Reduced grazing control is apparently an important and often overlooked factor for biomass accumulation of free-floating macroalgae under eutrophic conditions.

Submerged macrophyte seed bank in a Mediterranean temporary marsh: abundance and relationship with established vegetation

The abundance and composition of the submerged macrophyte seed bank in the Doñana marsh (southwestern Spain) was evaluated to assess its relationship with the overlying vegetation. The results obtained demonstrate the existence of a dense seed-bank, both in terms of the number of seeds and their biomass, which represented about 10% (5% for angiosperms and >20% for Charophyta) of the total plant biomass, which ensures the maintenance of the annual submerged macrophyte populations of the seasonally inundated Donñana marsh. Seed bank and established vegetation were coupled, as reflected in the existence of significant correlations between their structure and abundance. This coupling was lacking for Charophyta, whose oospores are widespread and abundant across the marsh, even at locations where they are absent, or rare, in the established vegetation. These differences between the seed bank of annual angiosperms and Charophyta appear to reflect, in part, structural differences between angiosperm seeds and Charophyta oospores, with important ecological consequences. An important aspect of these differences is the allocation of Charophyta reproductive effort to many small propagules lacking embryo storage, compared to those of angiosperms, which ensures their efficient dispersal and numerical abundance in the seed bank.

Phytoplankton, nutrients, and transparency in Danish coastal waters

We present a comparative analysis of 1400 data series of water chemistry (particularly nitrogen and phosphorus concentrations), phytoplankton biomass as chlorophylla (chla) concentrations, concentrations of suspended matter and Secchi depth transparency collected from the mid-1980s to the mid-1990s from 162 stations in 27 Danish fjords and coastal waters. The results demonstrate that Danish coastal waters were heavily eutrophied and had high particle concentrations and turbid waters. Median values were 5.1 μg chla 1−1, 10.0 mg DW 1−1 of suspended particles, and Secchi depth of 3.6 m. Chlorophyll concentration was strongly linked to the total-nitrogen concentration. The strength of this relationship increased from spring to summer as the concentration of total nitrogen declined. During summer, total nitrogen concentrations accounted for about 60% of the variability in chlorophyll concentrations among the different coastal systems. The relationship between chlorophyll and total phosphorus was more consistant over the year and correlations were much weaker than encountered for total nitrogen. Secchi depth could be predicted with good precision from measurements of chlorophyll and suspended matter. In a multiple stepwise regression model with In-transformed values the two variables accounted for most of the variability in water transparency for the different seasons and the period March–October as a whole (c. 80%). We were able to demonstrate a significant relationship between total nitrogen and Secchi depth, with important implications for management purposes.

Modelling advective transport of Ulva lactuca (L) in the sheltered bay, Møllekrogen, Roskilde Fjord, Denmark

Biomass, grazing, sporulation, and growth of the free-floating macroalgae Ulva lactuca were measured in a small bay of 420 000 m2 inside Roskilde Fjord, Denmark. The measurements where made at 10 stations with a spatial resolution of a few hundred meters, and a temporal resolution of two times per week. A two-dimensional transport-dispersion model of Ulva lactuca was set up for the area. Erosion of the macroalgae was modelled as a function of shear stress produced by horizontal- and orbital current velocity. The measured biomass abundance, grazing rates, sporulation rates and growth rates were used as input to the model, and two scenarios were simulated: one with and one without simulated transport-dispersion. Measured biomass were applied as initial values for each consecutive simulation period of 3–4 days. Simulation results exhibited significantly better agreement with measurements in the transport-dispersion scenario. This result strongly supports the working hypothesis, that advective transport is significant in controlling Ulva lactuca biomass, and that advective transport, therefore, must be included in a general model of Ulva lactuca dynamics.

The determination of the age and growth of SE Asian mangrove seedlings from internodal counts

The occurrence of periodic changes in the length of the internodes of the seedlings of SE Asian mangrove species (Rhizophora apiculata, Avicennia alba, and Sonneratia caseolaris) growing in the Pak Phanang estuary (Nakhon Si Thammarat, SE Thailand) was confirmed. The annual nature of these periodic changes was verified through comparisons with the observed average number of internodes produced by plants tagged for one year. The cycles in the sequence of internodal length of the seedlings had an average (± SE) period of 16.0 ± 0.78 internodes for Avicennia alba, 25.0 ± 1.2 internodes for Sonneratia caseolaris, and 8.29 ± 0.40 internodes for Rhizophora apiculata. These values are remarkably close to, and not significantly different (t‐test, P > 0.05) from the average (± SE) annual number of internodes of 17.6 ± 0.8 for Avicennia alba, 28.8 ± 2.1 for Sonneratia caseolaris, and 8.03 ± 0.36 for Rhizophora apiculata produced in a year by tagged plants. In addition, the average (± SE) growth of the tagged plants was 0.39 ± 0.018 cm day-1 for Avicennia alba, 0.24 ± 0.015 cm day-1 for Sonneratia caseolaris, and 0.091 ± 0.0036 cm day-1 for Rhizophora apiculata. These estimates were very close to those (0.45 ± 0.06 cm day-1 for Avicennia alba, 0.32 ± 0.01 cm day-1 for Sonneratia caseolaris, and 0.13 ± 0.005 cm day-1 for Rhizophora apiculata) derived as the product of the mean number of internodes produced per day, calculated from the inferred number of internodes produced per year, and the mean length of the internodes of the plants. These findings indicate that the use of regular, annual changes in the length of the internodes of mangrove seedlings can be used reliably to determine their age and to estimate their growth rate.

Significance of advective transport of Ulva lactuca for a biomass budget on a shallow water location

Ulva lactuca biomass, growth and grazing rates and drifting biomass was measured in the early summer period at a 1 m deep near-shore station in a sheltered bay in Roskilde Fjord, Zealand, Denmark. The expected biomass was calculated for each sampling date and compared with the measured. Regression analyses were performed between drifting biomass and the relative difference between measured and calculated biomass (R2 = 0.65, P < 0.054) and between wind velocities and drifting biomass (R2 = 0.42, P < 0.057). Although not significant at the 5% level, the results indicated that advective transport was significant for balancing the biomass budget of U. lactuca. It is therefore proposed that dynamic modelling of U. lactuca should be based on a transport/dispersion model.