Mar Biol (2007) 152:25–36 DOI 10.1007/s00227-007-0676-2
RESEARCH ARTICLE
Spatial patterns of macroalgal abundance in relation to eutrophication Dorte Krause-Jensen Æ Anne Lise Middelboe Æ Jacob Carstensen Æ Karsten Dahl
Received: 15 February 2006 / Accepted: 9 February 2007 / Published online: 11 April 2007 Springer-Verlag 2007
Abstract Based on a large monitoring data set from Danish coastal waters we tested the hypotheses: (1) The vertical pattern of algal abundance is regulated by exposure in shallow water and by light limitation towards deeper water, resulting in a bell-shaped abundance curve, with peak abundance located deeper at more exposed sites, (2) in deeper water, total algal abundance and abundance of perennial algae decrease along a eutrophication gradient while (3) abundance and relative abundance of opportunists increase. The vertical pattern of algal abundance showed a peak at intermediate water depths which was located deeper in more exposed areas and thus confirmed our first hypothesis. For algae growing from depths of peak
Communicated by M. Ku¨hl. D. Krause-Jensen (&) Department of Marine Ecology, National Environmental Research Institute, Vejlsøvej 25, 8600 Silkeborg, Denmark e-mail:
[email protected] J. Carstensen K. Dahl Department of Marine Ecology, National Environmental Research Institute, Frederiksborgvej 399, 4000 Roskilde, Denmark A. L. Middelboe Marine Biological Laboratory, University of Copenhagen, Strandpromenaden 5, 3000 Helsingør, Denmark J. Carstensen European Commission, Directorate General Joint Research Centre, Institute for Environment and Sustainability, Inland and Marine Waters Unit, TP 280, 21020 Ispra (VA), Italy
abundance and deeper, the study demonstrated that total algal abundance and abundance of perennials and opportunists at given depths decreased significantly along a eutrophication gradient and the relationships had high explanatory power (R2 = 0.53–0.73). These results confirmed our second hypothesis. By contrast, the relative abundance of opportunists responded solely to salinity and was largest in the most brackish areas, in contradiction to hypothesis three. The lack of coupling between eutrophication and relative abundance of opportunists arises because both opportunists and the entire algal community were light limited and their ratio therefore relatively insensitive to changing water clarity. The analyses indicated that algal abundance initially responded slowly to increasing eutrophication but showed a more marked response at TN concentrations of 35–40 lM. However, the existence of possible threshold nutrient levels demands further analyses.
Introduction Eutrophication is a major threat to marine submerged plant communities in temperate coastal waters. Increased nutrient richness stimulates the growth of epiphytic algae (Borum 1985) as well as phytoplankton growth and thereby reduces water clarity (e.g. Nielsen et al. 2002a). Reduced water clarity and increased epiphytic biomass force depth limits upwards (Duarte 1991; Nielsen et al. 2002b) and generally reduce vegetation abundance in deep, lightlimited waters (Duarte 1991; Dahl et al. 2001; Dahl and Carstensen 2005; Krause-Jensen et al. 2003). Opportunistic and perennial macroalgal species may respond differently
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to changes in nutrient- or light levels. Nutrient enrichment tends to stimulate the growth of opportunistic algal species and an increased biomass of opportunists may shade out the perennial species (Littler and Littler 1980; Steneck and Dethiers 1994; Duarte 1995; Pedersen 1995). The abundance of opportunistic algae is therefore likely to increase at the expense of perennial algae as a function of increased nutrient input. Disappearance of large, perennial and canopy-forming algal species is critical because they are important components of marine macroalgal communities in which they create multilayered structures and ensure a continuous dense algal cover. Disappearance of these species at high nutrient levels may therefore cause important shifts in macroalgal communities which may affect their function as protection-, food supply- and spawning area for many fauna groups and also change grazing pressure and/or metabolic pathways (Duarte 1995). Clear dose-response relationships between changes in marine submerged plant communities and increased eutrophication have, however, only been established for few response variables. Among these is the lower depth limit of seagrasses covering the soft substratum, while the lower limit of macroalgae on hard substratum has been a less successful variable because it is more difficult to determine precisely and may be limited by lack of substratum. The abundance of macroalgae at a given depth or composition of macroalgal communities are therefore likely to be more useful response variables than algal depth limits. The expected responses of algal abundance and community composition to eutrophication may, however, be blurred because other factors than eutrophication contribute to regulating the algae (Krause-Jensen et al. 2007). Physical exposure is an important source of variation creating difference in algal abundance with increasing depth. Wind- and wave exposure, desiccation and ice-scour may reduce macroalgal cover in shallow coastal waters. Based on observations for other macrophytes (e.g. Fonseca et al. 2002, Krause-Jensen et al. 2003), we expect that physical disturbance in shallow water combined with decreasing light towards deeper water result in a bell-shaped depth distribution of algae with maximum algal abundance at intermediate water depths, given that substratum conditions are equally suitable across depths. As a consequence we expect a higher sensitivity of macroalgal abundance to eutrophication when focusing exclusively on algae growing at the depth of maximum algal abundance and deeper. Algal abundance at specific water depths also vary due to e.g. seasonal or inter-annual differences in climatic variables such as insolation, precipitation or wind regime or due to differences in the proportion of hard stable substratum. Moreover, differences in salinity affect the
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diversity (Nielsen et al. 1995, Middelboe et al. 1998) and depth penetration (Pedersen and Snoeijs 2001) of macroalgal species and may therefore also affect algal abundance at specific depths (e.g. Krause-Jensen et al. 2007). These variabilities complicate spatial comparisons across sites that are sampled at different times of the year, in different years or at sites with different availability of hard substratum for colonisation or different salinity. With a large dataset it is possible to account for the variability of these factors. In this study we aim to evaluate if total macroalgal abundance and the abundances of perennial versus opportunistic species are related to nutrient concentrations, water clarity and salinity in Danish coastal waters. We use a comprehensive dataset from a national monitoring program covering a wide range of coastal ecosystems. We focus on the abundance of macroalgae in deep waters where exposure effects are minimal, accounting for variability due to water depth, sampling time, proportion of hard, stable substratum and local variations before relating algal variables to nutrient levels, water clarity and salinity. We hypothesize that (1) the vertical pattern of algal abundance is regulated by exposure in shallow water and by light limitation towards deeper water, resulting in a bellshaped abundance curve with peak abundance located deeper at more exposed sites, (2) at given depths in deeper water, total algal abundance and abundance of perennial algae decrease along a eutrophication gradient, while (3) abundance and relative abundance of opportunistic algae increase.
Methods Algal data We used algal cover data from the Danish National Monitoring and Assessment Programme and regional monitoring activities collected by the Danish counties and stored centrally in the National Environmental Research Institutes’ (NERIs) database. Data consist of 1,419 observations representing water depths between 1 and 13 m. Sampling took place along 1–11 depth gradients (sites) in each of 27 coastal areas (Table 1; Fig. 1). Algal data were collected during summer (May–September) of 2001 and 2003 according to common guidelines (Krause-Jensen et al. 2001), where divers visually recorded the percent cover of individual erect algal species and of the total erect macroalgal community on hard, stable substratum. Algal cover was estimated in percent of the hard stable substratum within sub-areas of 25-m2 seabed along the depth gradients (sites). Data sets where the summed cover of algal species constituted 13 m and we therefore restricted further analyses to water depths
