algae aquatic background bladderwrack canada coast littoral marine natural nature pacific plant seaweed shore texture vancouver rock sea block boulder coastal coastline cracked eroded erosion fissure fractured Gower Great Britain shoreline tide wales water brown green low olive Pembrokeshire rocks slimy united kingdom welsh beach face abstract blue floats haptera holdfast kelp lamina ocean planktonic pools saavem sorus spore stipe thallus tidal weed