DEPTH COLONIZATION OF EELGRASS (ZOSTERA MARINA) AND MACROALGAEAS DETERMINED BY WATER TRANSPARENCY IN DANISH COASTAL WATERS (2002)

Authors:

SØREN LAURENTIUS NIELSEN, KAJ SAND-JENSEN, JENS BORUM AND OLE GEERTZ-HANSEN

THE MAIN ACTORS

OUTLINE OF THE PAPER

A comparative analysis of lower depth limits for growth of eelgrass, large brown algae and other macroalga

162 transects in 27 Danish fjords and coastal waters

Coupled to 1400 dataseries of water chemistry (nitrogen) and Secchi depth

Why is it important to now how Secchi depth and nitrogen affect the lower limits of eelgrass and macroalgea?

INTRODUCTION

Eelgrass is a rooted macrophyte Eelgrass and macroalgea are important for

the structure and function of coastal marine ecosystem

Both macroalgea and eelgrass increase sedimentation

Eelgrass stabilise the sediment Refuge for fish and invertebrates

INTRODUCTION

Depth distribution of eelgrass and macroalgae decrease with increasing eutrophication

The main parameter limiting macrophyte depth colonization is light

Macrolagea can grow deeper than seagrass Probably due to less non photosynthetic tissue

Light penetration depth is meassured as Secchi depth

THE SECCHI DISC

SECCHI DEPTH

Correlates to 5-20% irradiance left Easy meassuring protocol Often used due to the convenience of the

method Secci depth is strongly dependent on nutrient

concentration Nitrogen is often the most important nutrient

controlling net primary production and Secchi depth in coastal marine areas

HYPOTHESIS

“Under the assumption that light penetration limits macrophyte depth colonization there will be a linear relationship between Secchi depth and maximum depth of colonization of macrophytes”

”Due to the higher proportion of non-photosynthetic tissue in eelgrass (e.g., roots and rhizomes), leading to relatively higher respiratory costs, we expect eelgrass to exhibit lower absolute depth limits”

Median: 3,6m

Median: 550ug N*l-1

Extremely high value for marine areas

RESULTS

RESULTS

Brown algea is also dependent on appropriate substrate

RESULTS

RESULTS

CONCLUSION

There is highly significant, simple linear relationships between Secchi depth and maximum depth of colonization for both eelgrass and macroalgae.

These relationships, although highly significant, show a relatively wide scatter and can only serve as models describing the overall trend and the relative change in depth limit of macrophytes occurring with a certain change in Secchi depth.