Anne Lise Middelboe

Long-term changes in macroalgal communities in a Danish estuary

Abstract Long-term studies of the composition of marine macroalgal communities are rare because of the lack of quantitative investigations in the past. In Denmark, a single quantitative study was performed in 1941–1943 in the nontidal estuary Isefjord–Roskilde Fjord, at several sites of variable salinity (10–22 ‰) and nutrient availability. This study was used as a reference for evaluating changes in the richness and relative abundance of macroalgal species from different taxonomic groups and functional forms at sites experiencing increasing concentrations of nitrate and light attenuation during the 50 years elapsed since then. Qualitative measures showed no, or few, changes over the period. In contrast, quantitative measures showed a significant increase in the relative abundance of small, opportunistic green algae (e.g. species of Chaetomorpha, Cladophora, Enteromorpha and Ulva) and a significant decrease in the relative abundance of large, perennial brown algae (e.g. Fucus serratus and F. vesiculosus). Along with these changes, the diversity of functional form groups declined. The inner parts of Roskilde Fjord, which were already nutrient-rich 50 years ago, did not show any significant changes in the composition of the macroalgal communities. The temporal changes during the 50 years are consistent with the spatial differences observed both then and now along the nutrient gradients. The results imply that quantitative measures of the abundance of macroalgal species having different thallus form, longevity and taxonomy are more sensitive and robust than qualitative measures in reflecting changes in the communities brought about by cultural eutrophication. The response of macroalgal communities gradually diminishes with progressively higher nutrient loadings, suggesting that the impact of eutrophication on coastal macroalgae will follow a geometric series similar to that established for lakes of different trophic levels.

Highly predictable photosynthetic production in natural macroalgal communities from incoming and absorbed light

Photosynthesis–irradiance relationships of macroalgal communities and thalli of dominant species in shallow coastal Danish waters were measured over a full year to test how well community production can be predicted from environmental (incident irradiance and temperature) and community variables (canopy absorptance, species number and thallus metabolism). Detached thalli of dominant species performed optimally at different times of the year, but showed no general seasonal changes in photosynthetic features. Production capacity of communities at high light varied only 1.8-fold over the year and was unrelated to incident irradiance, temperature and mean thallus photosynthesis, while community absorptance was a highly significant predictor. Actual rates of community photosynthesis were closely related to incident and absorbed irradiance alone. Community absorptance in turn was correlated to canopy height and species richness. The close relationship of community photosynthesis to irradiance is due to the fact that (1) large differences in thallus photosynthesis of individual species are averaged out in communities composed of several species, (2) seasonal replacement of species keeps communities metabolically active, and (3) maximum possible absorptance at 100% constrains the total photosynthesis of all species. Our results imply that the photosynthetic production of macroalgal communities is more predictable than their complex and dynamic nature suggest and that predictions are possible over wide spatial scales in coastal waters by measurements of vegetation cover, incoming irradiance and canopy absorptance.

PATTERNS OF MACROALGAL SPECIES DIVERSITY IN DANISH ESTUARIES

We analyzed species number of macroalgae in relation to environmental variables at two spatial levels comprising 202 individual sites and 26 entire estuaries in Denmark. The species number of macroalgae increased with salinity and declined with nutrient concentrations both at the sites and in the estuaries. Availability of hard substratum was associated with higher species richness at the sites. The number of macroalgal species in the estuaries increased with higher mean depth and longer coastline, suggesting that both the vertical and horizontal extension of the colonization area are important for the maximum representation of macroalgal species. Mean depth explained as a single predictor 60% of the variability in species number in entire estuaries. Estuaries with high mean depth also tend to be large and have high salinity and transparent waters due to efficient exchange with open waters. In conclusion, we find that the regulation of species richness of macroalgae in Danish estuaries, though complex, is influenced predictably by salinity, water transparency, nutrient concentration, and availability of hard substrata.

Direct effects of pH and inorganic carbon on macroalgal photosynthesis and growth

Abstract The maximum final pHs reached during extended photosynthesis were 8.83 for the red algae Delesseria sanguinea and 10.19 for the brown algae Fucus vesiculosus, as long as the concentrations of exceeded 1 mmol l−1, suggesting the existence of an upper pH limit for macroalgal photosynthesis. In experiments initiated with low concentrations of inorganic carbon, the final pH decreased with reduced initial pH, indicating that inorganic carbon became limiting. Effects of high O2 concentrations could be ruled out as a contributing factor in pH drift experiments initiated at or above pH 7.5. The growth rates of both species decreased when pH exceeded 8.5. However, although the growth of D. sanguinea stopped at pH 9.1–9.3, depending on acclimation, F. vesiculosus could maintain its growth up to a pH of 9.8 in these experiments. Additional experiments carried out on F. vesiculosus at two pH levels, 8 and 9.3, and a range of dissolved inorganic carbon concentrations revealed that the growth of F. vesiculosus...Abstract The maximum final pHs reached during extended photosynthesis were 8.83 for the red algae Delesseria sanguinea and 10.19 for the brown algae Fucus vesiculosus, as long as the concentrations of exceeded 1 mmol l−1, suggesting the existence of an upper pH limit for macroalgal photosynthesis. In experiments initiated with low concentrations of inorganic carbon, the final pH decreased with reduced initial pH, indicating that inorganic carbon became limiting. Effects of high O2 concentrations could be ruled out as a contributing factor in pH drift experiments initiated at or above pH 7.5. The growth rates of both species decreased when pH exceeded 8.5. However, although the growth of D. sanguinea stopped at pH 9.1–9.3, depending on acclimation, F. vesiculosus could maintain its growth up to a pH of 9.8 in these experiments. Additional experiments carried out on F. vesiculosus at two pH levels, 8 and 9.3, and a range of dissolved inorganic carbon concentrations revealed that the growth of F. vesiculosus was reduced at pH 9.3 compared with pH 8, irrespectively of inorganic carbon concentration. The results suggest that high pH may be an important, but so far overlooked, factor in regulating coastal primary production.

Spatial and interannual variations with depth in eelgrass populations

Abstract Measurements (1400) of eelgrass ( Zostera marina L.) shoot density and biomass along 19 depth transects in Oresund between Denmark and Sweden were analyzed during late summer in 4 years to characterize the spatial and temporal variation in shallow, intermediate and deep waters. The reduced physical harshness and reduced light availability with depth led to the hypotheses that: (1) variability in shoot density and biomass with depth is highest in shallow perturbed waters and in deep light-limited water and lowest at intermediate depths, and (2) spatial and temporal variabilities resemble each other because they are influenced by the same underlying causes. Spatial variances in both shoot density and biomass were significantly positively related to mean values at all depths and variances, therefore had to be compared based on relationships between variances and mean values. The hypotheses were supported by the findings that spatial and temporal variances in shoot density were both significantly highest in shallow water where high physical disturbance and high irradiance are conducive to high variability. However, we observed no significant differences in the variability of shoot density between intermediate and deep waters suggesting that the effects of reduced disturbance and the increased risk of severe light limitation at the depth boundary outweighed each other. Spatial variance in biomass did increase more with means in deep water than in shallow and intermediate waters, while temporal variance showed no differences with depth. Overall, spatio-temporal variability in shoot density and biomass relative to means declined at higher mean values probably because self-shading and space limitation set an upper boundary on eelgrass abundance. Wrong conclusions would have emerged if variances or coefficients of variation of shoot density and biomass with depth had been compared directly without correcting for the influence of different means.

Patterns of species number and abundance in macroalgal communities in coastal waters

Predictable patterns of species number have been observed in relation to habitat size, habitat heterogeneity and environmental conditions, while patterns in relative abundance of species have been examined for few communities and no assembly rules have been established. We studied communities of attached macroalgae in 61 individual sites located in four different areas; the inner, middle and outer parts of three neighbouring low-tidal estuaries and the adjacent open waters of the Kattegat, Denmark. The objectives were to determine (1) the relationships of species number and rank-abundance to the environmental conditions, and (2) the importance of scale and the consistency of species rank number at the sites for these relationships. We found that species number increased significantly from the inner estuaries to the open coastal waters along with decreasing nutrient concentrations. Turnover (β) diversity was lowest in the open waters suggesting that species composition was more similar among samples there than in the estuaries. Rank-abundance curves did not differ between depth intervals and individual sites across the environmental gradients. However, the summed rank-abundance patterns for two sites showed significantly steeper initial slopes and dominance of few species (i.e., low evenness) in the inner estuaries than in open waters. This pattern was due to high rank consistency of dominant species among sites in the inner estuaries. In open waters rank consistency was low, and the summed abundance across sites showed an even abundance of species. The results imply that the scale of the study and the community variability observed at that particular scale, is the main determinant of abundance patterns.