Sjøgras

Sjøgraset Posidonia oceanica ved Naxos i Hellas
Ålegras

Sjøgras eller sjøgress er blomsterplanter som har tilpasset seg et liv helt neddykket i sjøvann.

De omfatter omtrent 60 arter i familiene Hydrocharitaceae, Cymodoceaceae, Posidoniaceae, Zosteraceae og Ruppiaceae.[1] Alle tilhører ordenen Alismatales innenfor de enfrøbladete plantene. De er altså ikke gras i egentlig betydning. Navnet skyldes at de har lange og smale blad, og at de vokser i tette bestander som minner om enger under vann. Noen egentlige grasarter, som Spartina og saltgras (Puccinellia), vokser med røttene i sjøvann, men de regnes ikke som sjøgras.

Sjøgras er festet til bunnen med røtter som går ned i bunnsedimentene. De vokser derfor helst på bløtbunn, mens tang og tare dominerer på hardbunn. En kraftig jordstengel ligger horisontalt nede i sedimentene. Korte, vertikale skudd med blad og blomster sitter med jevne mellomrom på jordstengelen.[2]

Utbredelsen omfatter alle hav utenom polområdene. De er avhengig av lys for å drive fotosyntese og vokser derfor bare på grunt vann. Hydrogenkarbonat brukes som karbonkilde i stedet karbondioksid, som landplantene bruker. Bladene er smale og tynne for å gjøre det lettere å ta hydrogenkarbonat ut av vannet. De har lyktes i å utvikle pollinering under vann, men vegetativ formering har likevel stor betydning for disse plantene.

Områder med sjøgras er produktive økosystemer, og en mengde dyrearter lever her. Dyr som beiter på sjøgras er blant annet suppeskilpadde, sjøkyr, fisk, gjess, svaner, sjøpiggsvin og krabber.

Det eneste vanlige sjøgraset i Norge er ålegras, som finnes langs hele kysten på bløtbunn ned til 10 m dyp. I noen brakkvannsområder finnes det enger med havgras og tjønnaks. Enger med dvergålegras er sjeldne og finnes bare i Oslofjorden, Jæren og Sunnhordland.[3]

Referanser

  1. ^ Seagrass Recovery Besøkt 15. mai 2014.
  2. ^ N. Marbà m.fl. (1994). «Migration of large-scale subaqueous bedforms measured with seagrasses (Cymodocea nodosa) as tracers». Limnol. Oceanogr. 39 (1): 126–133. ISSN 1939-5590. doi:10.4319/lo.1994.39.1.0126. 
  3. ^ «Ålegrasenger». Miljødirektoratet. 29. januar 2010. Arkivert fra originalen 17. mai 2014. Besøkt 15. mai 2014. 

Litteratur

  • E.P. Green og F.T. Short, red. (2003). World Atlas of Seagrasses. University of California. ISBN 978-0-520-24047-6. 

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