At the mercy of the waves: Sandbanks below and above the waterline

When talking about the sea and the sand, the first thing that comes to mind are long, beautiful beaches full of clean, fine-grained sand washed by the waves. Under the surface, the landscape and conditions are much the same. However, despite its benevolent appearance, the almost continuous subtle motion of the sand creates a harsh habitat, where survival requires special qualities.

Although the sandy sea bottom looks quite deserted with only a quick glance, closer inspection reveals a number of invertebrates and fish living there, protected by the vascular plants which are rooted in the shallow sunlit zone.

Underwater sandbanks, on the other hand, refer mainly to mounds composed of sand, gravel, silt, and mud, which rise above the flat sandy seafloor. The biota of sandbanks is often similar to that of flat sandy bottoms, and the various species, in addition to the sediments, are also determined by the depth, the amount of light, the salinity, and the openness of the beach.

Most of the sandy bottoms are found in relatively shallow water, i.e. less than 20 metres, from which the finer sediments are removed by wave action. Water currents can also move the sand underwater, just like the dunes on a beach.

Plants make sheltered living spaces on the sandy seafloor

As a moving platform, sandbanks are difficult places for vegetation to grow, particularly in open areas. In more sheltered areas, the vegetation of the sandy bottoms can be highly diverse. The anchoring of aquatic plants on sandy bottoms requires strong roots or rhizomes that bind moving sediments in place.

Exposed and open sand bottoms are usually dominated by seagrass (Zostera marina) or low-growing, rough stoneworts (Chara aspera). Other plant species found on sandy bottoms also include ditch-grasses, horned pondweeds, pondweeds, and the bearded stonewort.

The rootstock or rhizomes of aquatic plants bind the moving sediment material in place, mitigate wave currents, and provide habitats for many invertebrates, such as worms, clams, and crustaceans. In turn, these invertebrates are food for fish species, such as flounder and gobies.

Crustaceans which can burrow rapidly into the sand, such as the sand digger amphipod (Bathyporeia pilosa) and the sand shrimp (Crangon crangon), also survive on the bare sandy bottom if they succeed in avoiding predators.

Eutrophication, dredging, and sediment dumping threaten sandbanks

Waters that are clouded by eutrophication reduce the potential growth area for plant-dominated sand bottoms, and clean sand bottoms are silted up by sedimentation from dredging and sediment dumping. Also, eutrophication causes a dramatic increase in the growth of filamentous algae. This extensive filamentous algal growth eventually becomes detached and sinks to the bottom to form algal mats that cover both vegetation and the clean sandy bottom, thereby depleting the available oxygen there.

A lesser threat is also the potential extraction of bottom sediments, such as sand and gravel. 

Species of sandy bottoms 

  • Seagrass (Zostera marina
  • Rough stonewort (Chara aspera
  • Ditch-grasses (Ruppia spp.) 
  • Horned pondweeds (Zannichellia spp.) 
  • Pondweeds (Stuckenia spp., Potamogeton spp.) 
  • Bearded stonewort (Chara canescens
  • Polychaete worms (Hediste diversicolor, Marenzelleria spp.) 
  • Clams (Macoma balthica, Mya arenaria, Mytilus trossulus, Cerastodermaglaucum
  • Flounder (Platichthys flesus)
  • Gobies (Gobiidae) 
  • Sand amphipod (Bathyporeia pilosa
  • Sand shrimp (Crangon crangon)