Kelp - NOAA's National Ocean Service

As on land, primary pro­duc­tion by pho­to­syn­thesis is the driv­ing force that sus­tains life in the sea. But un­like on land, where ve­get­a­tion dom­in­ates and trees grow to enorm­ous heights, the primary pro­du­cers in the mar­ine en­vir­on­ment are usu­ally minutely small. The closest things to gi­ants are the lar­ger al­gae (sea­weed) in the shal­low coastal wa­ters.

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Kelp beds and sea grass mead­ows

Pho­to­syn­thesis is the pro­cess by which green plants (and other or­gan­isms con­tain­ing chloro­phyll) use wa­ter, car­bon di­ox­ide (CO2) and the en­ergy of sun­light to pro­duce simple sug­ars. These be­come en­ergy stores or the basis of new or­ganic mat­ter. With very few ex­cep­tions, this primary pro­duc­tion is the only way to cap­ture new en­ergy to sus­tain life on earth.

In the mar­ine en­vir­on­ment, pho­to­syn­thesis only takes place in the lit­toral zone and the il­lu­min­ated sur­face layer of the open sea (the eu­photic zone). All other mar­ine or­gan­isms, in­clud­ing those of the un­lit zones and the deep ocean, are ul­ti­mately de­pend­ent on the growth of these primary pro­du­cers, bear­ing in mind that this may place a long way away and on a com­pletely dif­fer­ent times­cale.

Wa­ter is highly ab­sorb­ent of sun­light, and while cer­tain wavelengths pen­et­rate to greater depths than oth­ers, light is fully ab­sorbed at a depth of around 200-400 metres. Primary pro­duc­tion is there­fore lim­ited to the shal­low con­tin­ental shelves and the sur­face wa­ters of the open seas, which ac­count for no more than a small pro­por­tion of the total volume of the oceans.

Algal thallus
Mounted on the leaf-like algal thallus, gas-filled pneumatocysts ensure buoyancy and keep the algae upright in the water.

Kelp – green, brown and red

Close to the coast, the large sessile al­gae or kelps are of­ten sig­ni­fic­ant. Usu­ally they are at­tached to rocky to other hard sub­strates in the shal­low wa­ter. They an­chor them­selves to the ground with a hold­fast, a root-like or­gan, but quite un­like the roots of ter­restrial plants. They of­ten form dense com­munit­ies, some­times called 'forests', which ex­hibit a dis­tinct zon­a­tion. In the in­ter­tidal zone, green al­gae are pre­dom­in­ant. Be­low this zone, brown al­gae are found. These in­clude blad­der­wrack and knot­ted wrack, which rely on gas-filled pods for buoy­ancy and to stay up­right in the wa­ter.

Be­low the brown al­gae, red al­gae oc­cur in the low­est in­ter­tidal zone, and far be­low them come the large kelp spe­cies such as blade kelp or sea tangle. Every al­gae zone is a dis­tinct hab­itat type, with a typ­ical as­semblage of flora and fauna spe­cies.

The sessile al­gae com­prise green al­gae, which also oc­cur in fresh­wa­ter, as well as the al­most ex­clus­ively mar­ine brown and red al­gae. These have a world­wide dis­tri­bu­tion and, run­ning counter to the gen­eral trend, they also oc­cur in tem­per­ate and cold wa­ters in an abund­ance of dif­fer­ent spe­cies. As far as is known, the north-west Pa­cific and the trop­ical and sub­trop­ical west­ern At­lantic seem to be par­tic­u­larly spe­cies-rich, while south­ern Aus­tralia has a large num­ber of en­demic spe­cies, i.e. not found any­where else.

In wide areas of the tem­per­ate and sub­arc­tic re­gions, hard sea­floors be­low the low-tide line are covered in growth of very large brown al­gae, in jungle-like kelp forests. Ori­gin­ally the term 'kelp' was ap­plied to sea­weed ash, which was used in the 19th cen­tury as a source of soda, potash and iod­ine, among other ele­ments. Later, the term was ex­ten­ded to al­gae of the gen­era Mac­ro­cystis, Nere­o­cystis and Lam­in­aria them­selves. Where the al­gae reach to the wa­ter sur­face they are known as kelp forests. If sub­merged, they are called kelp beds. Typ­ic­ally they are found in the cold cur­rents of the At­lantic and Pa­cific. Oc­ca­sion­ally, they also oc­cur in trop­ical wa­ters, in up­welling re­gions where cold deep wa­ter is pushed to the sur­face.

Brown algae
Brown algae like the giant bladder kelp (Macrocystis pyrifera) can reach the colossal length of 100 metres. The long fronds grow by up to 60 centimetres a day. (Photo: NOAA)
The kelps' holdfast organs
The kelps' holdfast organs serve only for anchorage to the substrate, and unlike terrestrial plant roots, they play no part in feeding.

Di­versity in fig­ures: Biod­iversity of gi­ant al­gae (Source: John and Tittley, in WCMC (1992)

 Chlorophyta (Green Algae)
Order8
Genera170
Species1040
Habitatmostly marin
 Phaeophyceae (Brown Algae)
Order14
Genera265
Species1500
Habitatmarin
 Rhodophyta (Red Algae)
Order16
Genera< 555
Species< 2.500
Habitatmostly marin
 OrderGeneraSpeciesHabitat
Chlorophyta (Green Algae)81701040mostly marin
Phaeophyceae (Brown Algae)142651500marin
Rhodophyta (Red Algae)16< 555< 2.500mostly marin

But there are also free-float­ing sea­weed jungles. The largest and best known of these is the Sar­gasso Sea, an area of the At­lantic span­ning ap­prox­im­ately 8 mil­lion square kilo­metres, east of the Gulf of Mex­ico. In the warm wa­ter of the Gulf Stream, vast ex­panses of two red-brown al­gae spe­cies known col­lect­ively as gulf­weed (Sar­gas­sum natans and Sar­gas­sum fluit­ans), flour­ish and re­pro­duce ve­get­at­ively. Their total mass is es­tim­ated at 4–10 mil­lion tonnes live weight. The Sar­gasso Sea is par­tic­u­larly im­port­ant as the breed­ing ground of the European eel (An­guilla an­guilla) and as a hab­itat for the log­ger­head sea turtles (Car­etta car­etta) which hatch off the North Amer­ican coast.

Kelp forests and beds are highly pro­duct­ive and provide a spe­cial hab­itat for a host of mar­ine or­gan­isms. Des­pite the vast quant­it­ies of bio­mass avail­able, up to 100 tonnes of brown al­gae per hec­tare, re­l­at­ively few or­gan­isms feed dir­ectly on the liv­ing plants. No more than an es­tim­ated 10% of the al­gae bio­mass pro­duced finds its way into the food web via plant con­sumers. The vast ma­jor­ity only be­comes avail­able for con­sump­tion as de­tritus or dis­solved or­ganic mat­ter.

Red al­gae also play a part in the form­a­tion of coral reefs. These sup­port an al­gae flora of their own, which is gen­er­ally highly di­verse. It is likely that many more of the cal­cium-ex­cret­ing red al­gae in­volved in reef build­ing re­main to be dis­covered. The trop­ical and sub­trop­ical man­groves have their own typ­ical al­gae flora, which dis­tin­guishes them from the less spe­cies-rich, soft shal­low shores of the tem­per­ate zone. Nor­mally, sandy coasts have a smal­ler num­ber of spe­cies, and of­ten rep­res­ent a bar­rier to dis­tri­bu­tion.

Al­ter­a­tions to the shape of the coast, par­tic­u­larly the cre­ation of ar­ti­fi­cial 'rocks' such as har­bour moles or struc­tural found­a­tions, can bring about local in­creases in al­gae spe­cies di­versity by cre­at­ing ad­di­tional hab­itat area along sandy coasts. On the other hand, mar­ine pol­lu­tion, par­tic­u­larly of sheltered bays, man­grove forests and coral reefs, has caused a de­cline in spe­cies and pro­moted more re­si­li­ent spe­cies to the det­ri­ment of oth­ers.

Photo: University of Southern Mississippi Gulf Coast Research Laboratory.
Free-floating Sargassum offers protection and habitat for many species.
Sea grass meadows
Light sea grass meadows in the shallow waters of the Florida Keys. (Photo: Paige Gill, NOAA)

Green underwater meadows

There are ap­prox­im­ately 45 known sea grass spe­cies in the world, and these are clas­si­fied as vas­cu­lar plants. Al­though sea grasses do not reach such im­press­ive sizes as kelp, in fact their dense growth and in­tric­ate root sys­tems help to sta­bil­ise the coastal ocean floor. They en­cour­age sed­i­ment­a­tion, bind many nu­tri­ents in their leaves and root sys­tems, and provide hab­itat for a rich bot­tom fauna. Most spe­cies pro­duce flowers which are fer­til­ized by means of float­ing pol­len.

The sea grasses of­ten sup­port large pop­u­la­tions of epi­phytic (plant hos­ted) or periphytic (sur­face cling­ing) al­gae and small an­im­als. These in turn are an im­port­ant source of nour­ish­ment for plant-eat­ing fish and other mar­ine or­gan­isms.

In the 1930s, stocks of the large sea grass (Zostera marina) on both sides of the At­lantic were des­troyed by a fungal dis­ease. This gave rise to dev­ast­at­ing im­pacts in quite dif­fer­ent loc­a­tions. Within a year, stocks of the Brent goose ( Branta bernicla) had fallen to around ten per­cent of their pre­vi­ous pop­u­la­tion dens­ity. This co­in­cided with the loss of the shal­low wa­ter sea grass mead­ows – which were, at the time, the most im­port­ant food plants in their over­win­ter­ing ter­rit­or­ies. Bi­valves, mol­luscs and crabs dis­ap­peared com­pletely from vast areas, and fish stocks de­clined.

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