Marine plants – Introduction

Marine plants can only live in the upper, illuminated (‘euphotic’) layers of the sea. In clear waters, they may be found down to a depth of around 200 meters; however, most species are restricted to much shallower water, often to the tidal zone.

A great variety of algae grows close to the waterline.

A. Seagrass

The two most common species of seagrass in our region are Cymodocea nodosa, which occurs in sandy bays and grows in rather shallow water, and the Neptune grass (Posidonia oceanica), which forms dense meadows in greater depths. Seagrass meadows are among the most important and productive biotopes in the sea and provide shelter and food for countless animals.

Cymodocea nodosa

Neptune grass (Posidonia oceanica)

seagrass meadow

B. Algae

Algae make up the vast majority of marine plants. They are an extremely interesting and diverse group of plants, the classification of which is still only partially understood. Identifying the species is often challenging, as many characteristics can only be seen with a powerful magnifying glass or a microscope. Identification is also made difficult by the fact that there is little suitable literature available. However, the fact that I cannot identify all species, or that some identifications may be incorrect, should not be a hindrance to presenting our common species. Especially because algae are truly fascinating!

The different species of algae exhibit a wide variety of growth forms.

The marine algae are divided into two large groups: the planktonic algae, which float freely in the sea, and the benthic algae, that grow on the seabed. Planktonic algae play a particularly important role in the marine ecosystem as primary producers: they produce oxygen, bind carbon dioxide, release organic substances into the sea water and serve as food for countless marine animals. The pelagic habitat they live in is much larger than the benthic habitat: it encompasses the upper water layers of the entire sea. The most important groups of planktonic algae are the dinoflagellates, the single-celled, bizarre diatoms and the chrysophyta, which are also mostly single-celled and often have a siliceous or calcareous skeleton. However, all these species are difficult to study due to their occurrence in the open sea and their tiny size, so unfortunately we have to leave them out of consideration here.

In contrast to the pelagic habitat, the euphotic zone of the sea floor where the sessile benthic algae grow, is limited to a more or less narrow strip along the coast. Benthic algae play a lesser role as a food source in the sea than planktonic algae, as they protect themselves quite effectively from being eaten by means such as calcium deposits or inedible substances. However, they provide the living space for numerous microscopic and larger animals. Many species grow in the eulittoral (the tidal zone) and can be studied right at the coast.

typischer Algenbewuchs auf Schieferfelsen

typical alga communities at the coast, in the first picture on slate, in the second on marble

The benthic algae belong to three large groups: green algae, brown algae and red algae. These three groups differ considerably from one another in terms of anatomy, chemical composition and ways of reproduction. At our coasts, representatives of all these groups can be found.

Below, I present each group with a few representative photos; clicking on the heading will take you to the corresponding page.

(click on the images to view as slideshow)