Organism - Sediment Relationships on a Modern Tidal Flat

Sandf la t s i n t h e pro tec ted Bodega Harbor a r e inhab i t ed by a d i v e r s e i n f a u n a l assemblage (over s p e c i e s ) c o n s i s t i n g predominantly of molluscs , chae tes and o t h e r worm-like animals , and crus taceans . Sediment g e n e r a l l y is w e l l s o r t e d , f i n e t o medium grained sand wi th smal l amounts of s i l t and c lay . Although r i p p l e s a r e formed by wind-driven and t i d a l c u r r e n t s , microcrosslamination and o t h e r p h y s i c a l sedimentary s t r u c t u r e s are later o b l i t e r a t e d by intense b i o t u r b a t i o n . On t h e upper t i d a l f l a t t h e t h a l a s s i n i d shrimp Cal l i anassa c a l i f o r n i e n s i s i s an a c t i v e depos i t f eede r . It produces predominantly staghorn-shaped burrow networks which extend 40 cm o r more below t h e sediment-waterinterface. The deposi tfeeding b iva lve s e c t a inhabits t h e middle s a n d f l a t and is a l s o an important t u r b a t o r . On t h e lower f l a t t h e phoronid Phoronopsis v i r i d i s is abundant, producing dense c l u s t e r s of vert i c a l tubes . I n genera l suspension feeder s l i v e more deeply w i t h i n t h e sediment than do depos i t feeders. Deposit f eede r s a r e abundant throughout t h e f l a t ; they tend t o burrow more deeply i n sediment of t h e high f l a t than i n t h a t of t h e low f l a t . Gradients i n s p e c i e s abundances from mean lower lower water t o h igher areas on t h e f l a t suggest t h a t phys ica l f a c t o r s such as exposure t i m e are important c o n t r o l s of t h e s p e c i e s ' d i s t r i b u t i o n s . However, exper imental f i e l d s t u d i e s i n Bodega Harbor have shown t h a t e c o l o g i c a l i n t e r a c t i o n s between s p e c i e s may a l s o a f f e c t d i s t r i b u t i o n s (Ronan, 1975). Presence of phoronids i n g r e a t abundance p r o h i b i t s attainment of normal burrowing depths by M. s e c t a , and i n a r e a s of over l ap between phoronids and Cal l i anassa c a l i f o r n i e n s i s , t h e r e i s a high incidence of broken and d i s tu rbed phoronid tubes . Thus b i o l o g i c a l a c t i v i t y may a f f e c t , d i r e c t l y o r i n d i r e c t l y , t h e d i s t r i bu t ions of i n f a u n a l animals. The same may a l s o have been t r u e i n t h e p a s t , and ichnocoenoses should be viewed as r e f l e c t i n g i n t e r a c t i o n s between animals a s w e l l as between t h e animals and t h e i r p h y s i c a l environment. INTRODUCTION Phys ica l S e t t i n g Bodega Harbor i s a shal low c o a s t a l embayment loca ted i n southwest County about 65 miles n o r t h of San Franc i sco (Fig. 1 on p. 1 ) . The n e a r l y landlocked embayment is s h e l t e r e d from p r e v a i l i n g nor thwes te r ly winds by a rocky pen insu la , Bodega Head, and a beach and dune-covered neck of land t o t h e n o r t h t h a t a t t a c h e s i t t o t h e mainland (Fig. 1 ) .