II. Overview of Beach Meiofauna and Macrofauna
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.
Next - Section III. Beach Zonation
Sandy Beaches Table of Contents
