THE AIR-SPORA NEAR THE EARTH S SURFACE 



Although the air-spora over the sea is clearly largely of land origin, 

 Petri dish sedimentation tests are difficult to interpret and cultural and 

 volumetric work is needed on the contribution from the ocean itself — 

 both in the zone of littoral influence studied by ZoBell, and far from shore. 

 The mode by which the ocean purifies the air flowing over it, and the fate 

 of airborne spores trapped by the ocean, are still obscure. 



Earlier workers, including Miquel and B. Fischer, found marine air 

 almost free from pollen, but this freedom is now seen to be only relative. 

 Erdtman (1937) operated a vacuum-cleaner filter trap at the masthead of 

 the M.S. Drottningholm during a voyage from Gothenburg to New York 

 extending from 29 May to 7 June, 1937. Compared with the average of 

 180 pollen grains per cubic metre recorded during spring at Vasteras 

 (no km. west of Stockholm), he found only o-i8 per cubic metre in the 

 North Sea, and 0-007 P^^ cubic metre in mid-ocean, with an increase 

 again on approaching North America. Temporary higher concentrations 

 ('pollen rains') occurred three times: oi Pinus (0-13 per cubic metre) in 

 the North Sea; of Almis viridis (0*045 P^^ cubic metre) and Cyperaceae 

 (o-oo6 per cubic metre) at a distance of 250 to 600 km. off" Newfoundland; 

 and of combined grasses, Plant ago^ and Rumex (totalling o-i per cubic 

 metre) at 220 to 300 km. from Nova Scotia and Massachusetts. During 

 strong western and north western winds, about mid way between 

 Iceland and Ireland, Erdtman caught tree pollens {Alnus, Betula^ Coryliis^ 

 Jiiniperus, Myricci^ Picea, Pinus, Popiilus, Qiiercus^ Salix, Tilia, Ulmus) 

 and herb pollens (Chenopodiaceae, Cruciferae, Cyperaceae, Ericaceae, 

 Gramineae, Plantago^ Umbelliferae, and Urtica)^ as well as spores of 

 Dryopteris and Lyco podium clavatiim. 



Erdtman's volumetric sampling firmly establishes the occurrence of 

 pollen in small but measurable concentration near the surface of the sea 

 right across the Atlantic, and there is no reason to doubt that the land air- 

 spora extends to all parts of the globe. Confirmatory evidence comes from 

 Transatlantic sampling by Dyakowska (1947) and Polunin (cf. 1955). 



Bishop Rock Lighthouse stands on a low rock at the southwestern 

 extremity of the Scilly Isles, which are a group of small, rocky islands 

 with few trees (mainly Ulmus and Pinus). Gravit}-' slide sampling on the 

 lighthouse platform 38 metres above sea-level (Hyde, 1956) showed mainly 

 pollen of Betula^ Qiiercus, and Fraxinus, with some Phtus. The total tree- 

 pollen deposit was quite large (2,800 per sq. cm. per year, compared with 

 2,000 at Aberdeen and Brecknock Beacons, and 10,000 at Cambridge). 

 The proportion of tree pollen of Bishop Rock was 27 per cent, and this is 

 t}''pical of country areas in Britain (in to\Mis it may reach 50 per cent). 

 It is remarkable that the greatest deposition of grass pollen recorded 

 for any centre in Britain during Hyde's gravit}^ slide survey was 1,679 

 grains per 5 sq. cm. at Bishop Rock on 29 June 1953. Whether this 

 resulted from high concentration, or from high efficiency of turbulent 

 deposition in strong winds, is not yet clear. 



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