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Historical changes in Zostera spp. on Strangford Lough. Alex Portig Quercus – The Queens University of Belfast. Zostera marina (Kircubbin Bay). Zostera marina (subtidal) perennial. Zostera marina (intertidal) perennial. Zostera marina var. angustifolia (intertidal) annual.
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Historical changes in Zostera spp. on Strangford Lough Alex Portig Quercus – The Queens University of Belfast.
Zostera marina (Kircubbin Bay) Zostera marina (subtidal) perennial
Zostera marina (intertidal) perennial
Importance of Seagrasses Primary production - Highly productive ecosystem Production = 2.7 g DW m-2 per day (Tropical forest 5.2 / Macroalgae 1.0) (Duarte, C.M. & Chiscano C.L. 1999 Aquatic Botany 65: 159-174) Other roles: • Provision of food for coastal food webs • Carbon sequestration from the atmosphere • Sediment stabilisation • Prevention of sediment re-suspension • Improvement of water transparency • Wave attenuation • Shoreline protection • Increased Biodiversity • Trapping and cycling of nutrients (Duarte, C.M. 2002 Environmental Conservation 29 (2): 192 – 206)
Records of Zostera distribution - Strangford Lough • First detailed records Lynn 1936 • Bleakley 1971 • Corbett 1980 • Portig 1994 • No detailed records in between these times. • Late 1990s to present – research by QUB/Quercus funded by DARD (Rivers Agency) and EHS.
1930’s Based on Lynn’s descriptions and maps • Zostera marina (perennial) was widespread in the sub and intertidal and had declined dramatically in recent years. • Reasons for decline – has since been established as a wasting disease caused by a marine slime mould (Labyrinthula) • This occurred across the Atlantic seaboard and wide scale loss of Zostera marina on Strangford resulting in the break down of mud banks and sediment shift. • Zostera noltii considered rare. • Zostera marina var. angustifolia present.
Records of Zostera marina on Strangford Lough – based on Lynn 1937
Relative Area 1970 - 100% 1980 - 49% 1990 - 30% 1970 – From Bleakley 1980 – From Corbett
1970 Based on Bleakley’s descriptions and maps • Zostera noltii dominant. • Zostera marina var. angustifolia present. • Widespread in the intertidal. • Patchy distribution in bays of Ardmillan and Whiterock. • No mention of Zosteramarina in intertidal.
1980 Based on Corbett’s descriptions and maps • Zostera noltii still dominant. • Zostera marina var. angustifolia present. • More restricted in distribution. • Loss chiefly from lower intertidal and from bays around Ardmillan and Whiterock. • No mention of Zosteramarina in intertidal.
Early 1990’s • Zostera noltii still dominant. • Zostera marina var. angustifolia present. • Further restriction in its distribution. • Density considered low. • No Zostera marina in the intertidal found.
Amount of Zostera calculated from counts of wildfowl (underlying trend). Using WEBS Data Blue line = raw data: Red line = underlying trend
Early 2000’s • Zostera noltii still dominant. • Zostera marina var. angustifolia more widespread. • Limited expansion in distribution. • Density improved. • Zostera marina found in the intertidal !. • This was most apparent in the north end of the Lough.
North end (1994) North end (1994)
Brent Geese Total Birdmonths – North End Data WWT/NT - WEBS
Causes of change? • Decline in 1930’s • Caused by wasting disease • Widescale loss of Z. marina from the intertidal • Likely to be also the case from the subtidal. • Not known if recovery occurred but by 1970 was absent from the intertidal. • Changed environmental conditions?
Possible causes 1970 to present • Sediment stability ? • Competition with other species ? • Climate ? • Water clarity ? • Long term cycles ?
Redrawn from Hemminga & Duarte 2000 Sediment Stability
Browned Zostera noltii and Zostera marina var. angustifoliaStrangford Lough 2002
Redrawn from: Kemp et al 2000
Redrawn from: Kemp et al 2000
The likely scenario ? • The restriction in distribution seen from 1970 to early 1990s most likely caused by reduced water clarity. • This is not to say that other factors do not play a role but are more likely to be localised. • Improvement over recent years is consistent with improved water clarity.