The high density and biomass of invertebrates in surf zone waters offers abundant food for juvenile fish like pampo (Trachinotus), mullet (Mugil), white kingfish (Menticirrhus), and silverside (Odontesthes bonariensis) and the shallow waters provide protection from larger predators, like rays, small sharks, and marine mammals. In the fine-grained sand (0.12-0.25 mm) of intertidal beaches and the swash zone abundant clams (Mesodesma mactroides and Donax hanleyanus), polychaetes (Spio gaucha), and crustaceans (Emerita brasiliensis and Excirolana armata) follow the advance and retreat of seawater. They feed on organic detritus and phytoplankton in surf zone waters, especially on dense brown patches of the diatom Asterionellopsis glacialis. Blooms of potentially toxic dinoflagellates (Gyrodinium, Dinophysis acuminata) occasionally cause massive mortality of clams. At low water, intertidal invertebrates are a vital food source for many resident and visiting shorebirds, like sandpipers (Haematopus and Calidris) and plovers (Pluvialis and Charadrius). Gulls (Larus dominicanus) are among the most numerous birds on the beach and together with occasional herons (Egretta thula, Ardea cocoi) and hawks (Milvago chimango, Polyborus plancus) forages throughout the year on stranded debris. Since migrating birds find roosting places within easy reach of feeding areas on the lonely beaches, the conservation of southern Brazilian coastal environments is of worldwide significance.
The impact of wave and wind energy on beach-dune sand transport and on foredune stability induces rigorous conditions and influences the composition of the herbaceous backshore and foredune vegetation. The backshore is a seasonally unstable environment due to erosive storm overwash and is exclusively colonized by the perennial rhizomatous Blutaparon portulacoides which capture windblown sand and forms up to 1 m high incipient embryo dunes. The predominantly terrestrial conditions favor a fauna of beetles (Bledius, Cicindela), though marine species like the ghost crab Ocypode quadrata also occur. Backshore species feed principally on detrital matter at the drift line.
Much of the sand flow from the beach cannot be stopped in embryo dunes and is deposited above the influence of storm tides by monospecific stands of the grass Panicum racemosum. The plant's clonal spreading and vertical growth promote efficient sand deposition (70 cm year-1), which counteracts sand erosion during periods of prevailing offshore winds and leads to the formation of 4-6 m high dune ridges. At the foot of these dunes, annual species like sea rocket (Cakile maritima) and Calycera crassiflora occasionally escape the rigors of backshores. Over time and with distance from the sea Panicum racemosum populations change from "invader", through "mature", to "regressive" stages, expressed by a gradual reduction in population density in response to sand deposition and nutrient input. On dunes with reduced sand accretion, Gamochaeta americana and Senecio crassiflorus may locally become dominant. Rodents (Ctenomys flamarioni and Calomys laucha) are common to drier frontal dunes where they ingest seeds and plant tissue. The foredune plant cover and dry substrate also offers niches for a large number of insects (>40 coleopterans) which are preyed upon by sand toad (Bufo arenarum arenarum), sand frog (Pleurodema darwini), and lizard (Liolaemus occipitalis).
Behind frontal dune ridges dry and seasonally inundated wet slacks with increased substrate stability and water table proximity display high plant and animal diversity. The endemic, perennial grass Andropogon arenarius, together with Spartina ciliata, Androtrichum trigynum, and Hydrocotyle bonariensis, characteristically occupies dry slacks. Humidity gradients between dry and wet slacks cause a change from Andropogon to Androtrichum trigynum in winter-inundated wet slacks. Co-dominants of wet slacks are Imperata brasiliensis, Hydrocotyle bonariensis, Baccopa monieri, and marsh remnants (Juncus acutus, Typha domingensis). Plants of sheltered slack areas provide protection and food for a diverse animal community. Among the insects, ants are the most conspicuous species. Other slack dwellers are rove beetles, weevil (Listroderes uruguayensis), wasps (Tachytes ornatipes), pigmy mole cricket (Neotridactylus carbonelli) and carabids. At night, the burrowing owl (Athene cunicularia), skunk (Conepathus chinga), armadillo (Dasypus hybridus), hare (Lepus capensis), and fox (Dusicyon gymnocercus) undertake feeding incursions into foredunes.