Which resource limits coastal phytoplankton growth/ abundance: underwater light or nutrients?

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Introduction

Marine life is based on single-celled algae drifting in the water column and visible only under a microscope. In 1887 Victor Hensen termed them phytoplankton. Marine phytoplankton is the food basis for e.g. small crustaceans like zooplankton as well as for filter feeders such as mussels and oysters, which are then eaten by larger animals like fish or wading birds. Phytoplankton requires light, carbon dioxide, and nutrients for its growth. Large rivers (e.g. IJssel, Rhine, Elbe) provide high amounts of nutrients to the North Sea. Riverine nutrient loads have increased up to eight-fold from 1950 till the late 1970s (van Beusekom, 2005[1]) due to usage of artificial fertilizers in agriculture, phosphorus in washing powder and increased urban population size. After their peak in the mid 1980s riverine nutrient loads significantly decreased due to revised environmental laws, including international agreements and improved treatment plants (e.g. Colijn and Garthe, 2005[2]) and have reached levels of the late fifties for phosphate. However, nitrogen loads did not decrease that much due to diffuse sources, especially from agriculture. Nutrient enrichment can stimulate algal production and biomass accumulation, leading to anoxia and large-scale mortality of fish and shellfish (Rosenberg and Loo, 1988[3]), and alter phytoplankton species composition so that the whole coastal food web could be impacted (Lancelot et al., 1987[4]; Reise and Siebert, 1994[5]). An example of such a change in species composition was the strong increase in the bloom density of Phaeocystis in the Dutch coastal waters, which due to decreasing nutrient concentrations are returning to pre-eutrophication levels (Kuipers et al., submitted). However, coastal areas differ in their response to nutrient enrichment, since especially in highly turbid waters underwater irradiance will limit phytoplankton growth strongly. This coupling complicates the development of management strategies for coastal areas, since reduced riverine nutrient discharges might reduce the food supply, for example for mussels, fish, wading birds and mammals, and therefore may lead to contradictory positions for those who want to exploit coastal systems. To analyse the current state of the phytoplankton a simple method is presented which enables an estimate of the limiting factors.

Figure 1:Contour plot of resource limitation at the Island of Sylt (List Tidal Basin) calculated following the index of Cloern (1999[6]). Blue fields indicate light limitation, yellow fields show nutrient (nitrogen) limitation, white fields indicate co-limitation of both resources. Crosses indicate data points. Missing data were interpolated by kriging. Comparing the 1985-1991 period with the 1999-2005 period shows that nitrogen limitation during summer has significantly increased (Loebl et al., 2008[7]).

Methodology

A useful index to indicate ecosystem sensitivity to eutrophication effects Cloern (1999[6]) presented a simple index that indicates the sensitivity of coastal ecosystems to eutrophication effects by assessing whether phytoplankton growth is light or nutrient limited. This index was developed from empirical data. Its application only requires the following parameters: water depth, global irradiance, water column irradiance (calculated from Secchi-depth or suspended matter content) and nutrient concentrations (N, P, and Si). Most parameters have already been included in monitoring programmes along the European North Sea coast for many years. The index provides a useful tool to estimate phytoplankton responses to changed anthropogenic nutrient supply. Applying the index of Cloern to European*Monitoring Data The index of Cloern (1999[6]) was used to estimate resource limitation at several coastal areas along the European continental coastal zone between 1985 and 2005 (Loebl et al., 2008[7] & submitted[8]). Light limitation was tested against limitation of the dissolved nutrients nitrogen, phosphorus or silicate on a monthly basis.

Figure 2:Contour plot of resource limitation of phytoplankton growth at the Island of Helgoland and at Büsum/Meldorfer Bucht calculated following the index of Cloern (1999[6]). Blue fields indicate light limitation, yellow fields show nutrient (plot a: nitrogen, plot b: phophorus) limitation, white fields indicate co-limitation of both light and nutrients. Crosses indicate data points. Missing data were interpolated by kriging. At Helgoland nutrient limitation of nitrogen and phophorus during summer increased significanttly between 1990 and 2005 (Loebl et al., 2008[7]).

Outcome

Resource limitation along the European continental North Sea coast Applying Cloern’s method to European coastal monitoring data showed that each investigated area exhibits its own limitation pattern, whereby light limitation during winter was a common feature of all sites (Fig. 1, 2). The List Tidal Basin (Island of Sylt) showed the longest period of nutrient limitation during summer, whereas phytoplankton growth at Büsum (Meldorfer Bucht) was limited by light during the whole year (Fig. 2). A significant increase of nutrient limitation in spring/summer with- in the past 15-20 years was observed at Sylt and Helgoland (Fig. 1 and 2), at Sylt presumably due to reduced riverine nutrient inputs. At Helgoland also altered hydrographical forcings might have changed nutrient levels. However, a future reduction of riverine nutrient loads could affect sites like Sylt more than turbid sites like the Meldorfer Bucht. In the Dutch coastal zone, strongly influenced by high nutrient loads from the River Rhine, a clear increase in nutrient limitation was observed, as was also the case in the Marsdiep area. Cloern's index provides a useful tool to get a first answer to the question which resource might limit phyto- plankton growth and how changing nutrient loads might influence coastal food webs.


References

  1. van Beusekom, J.E.E. (2005). A historic perspective on Wadden Sea eutrophication. Helgoland Marine Research, 59, 45-54.
  2. Colijn, F. & Garthe, S. (2005). EU Directives and their effects on the ecosystem of the Wadden Sea. Proceedings from the 11. Scientific Wadden Sea Symposium Esbjerg, Denmark, *4.-8. April 2005. Monitoring and Assessment in the Wadden Sea. NERI Technical Report 573, 13-20.
  3. Rosenberg, R. & Loo, L.O. (1988). Marine eutrophication induced oxygen deficiency: effects on soft bottom fauna, western Sweden. Ophelia, 29, 213-225.
  4. Lancelot, C., Billen, G., Sournia, A., Weisse, T., Colijin, F., Veldhuis, M.J.W., Davies, A. & Wassman, P. (1987). Phaeocystis blooms and nutrient enrichment in the continental coastal zones of the North Sea. Ambio, 16, 38-46.
  5. Reise, K. & Siebert, I. (1994). Mass occurrence of green algae in the German Wadden Sea. Deutsche Hydrographische Zeitschrift, Supplement, 1, 171-180.
  6. 6,0 6,1 6,2 6,3 Cloern, J.E. (1999). The relative importance of light and nutrient limitation of phytoplankton growth: a simple index of coastal ecosystem sensitivity to nutrient enrichment. Aquatic Ecology, 33, 3-16.
  7. 7,0 7,1 7,2 Loebl, M., Colijn, F. & van Beusekom, J.E.E. (2008). Increasing nitrogen limitation during summer in the List tidal basin (northern Wadden Sea). Helgoland Marine Research, 62 (1), 59-65. doi: 10.1007/s10152-007-0089-0.
  8. Loebl, M., Colijn, F., Beusekom, J. E. E. van, Baretta-Bekker, J. G., Lancelot, C., Philppart, C. J. M., Rousseau, V. & Wiltshire, K. H. (submitted). Recent changes in phytoplankton limitation patterns along the Northwest European continental coast - Consequences of nutrient reduction measures.



The main author of this article is Colijn, Franciscus
Please note that others may also have edited the contents of this article.


The main author of this article is Van Beusekom, Justus
Please note that others may also have edited the contents of this article.


The main author of this article is Wiltshire, Karen Helen
Please note that others may also have edited the contents of this article.