Sand beaches are very harsh environments, encompassing most of the rigors of the rocky intertidal (high wave action, wide temperature range, periodic tidal exposure) with the addition of high abrasion levels and lack of firm substrate for attachment. Beach fauna exhibit the characteristics of communities in harsh environments, namely low species diversity but large numbers of individuals of each species.
Central California beaches show classic patterns of beach zonation, first described by Dahl (1952) from beaches in Europe and South America. He recognized three zones, limited by the amount of tidal inundation: 1) subterrestrial fringe (Talitrid-Ocypodid belt); midlittoral zone (Cirolanid isopods); and the sub-littoral fringe (rich and varied fauna). In central California, it is appropriate to add an additional zone, the swash zone, which is dominated by the sand crab Emerita analoga (Oakden and Nybakken 1977; Figure 1). Good general references include Ricketts et al. (1968) for habitats, and Morris et al. (1980) & Smith and Carlton (1975) for animals.
Because meiofauna are a distinct fauna from the more obvious macrofauna, The distribution of meiofauna (organisms inhabiting the interstitial spaces between the sand grains) is strongly influenced by the grain size of the sand. If there is a significant silt component in the sediment, the interstitial spaces are filled by the silt particles, impacting the interstitial fauna. While most sand beaches have a strong enough wave regime to remove silt, there can be significant fine-sediment components near river mouths, at the base of landslides, and in relatively sheltered areas around sloughs and estuaries, reducing the abundance and variety of interstitial fauna.
Meiofauna are also influenced by tidal level. They exhibit pronounced vertical migrations within the sand, closely following the tidal curve. Near river mouths and other freshwater sources, the abundance of meiofauna can be impacted by sub-surface freshwater flows. Meiofauna distribution is also influenced by the depth of the anoxic layer (generally very deep on sand beaches), and organic content of the sand.
Few studies exist on meiofauna in central California. Narine (1976) found representatives of several phyla on a sand beach at Moss Landing (Table 1). Polychaetes, oligochaetes, and nematodes are dominant, with oligochaetes generally found in the higher intertidal. There are also seasonal pulses of harpacticoid copepods and archiannelid worms. Locy (1981) studied 2 species of oligochaetes at Moss Landing, Roe (1993) studied a nemertean, and Gowing (1981) studied an archiannelid. A species of tardigrade was described from the Hopkins beach in Pacific Grove (MacGinitie, 1969). There is also an interstitial diatom population which has not been studied (pers. obs.).
Despite the barren appearance of sand beaches, they harbor a diverse and abundant assemblage of macrofauna. These animals generally live buried in the sand, and are highly mobile, so are somewhat more difficult to study than, for instance, the more sessile organisms of rocky intertidal zones. Because the beach is a physically rigorous environment, physical factors often limit the distribution of these organisms. The beach can be divided into zones (see III. Beach Zonation), determined by tidal height, each with a distinct group of physical characteristics and animals.
Section I. Beach Structure and Formation