The Patos Lagoon Estuary, Brazil
171
to environmental characteristics. The deep channels (>6 m) have low
macrobenthic abundance and diversity but are an important route for
many migratory organisms. The subtidal soft-bottoms between 1.5- and
5-m depths offer spatial and trophic niches for dense populations of the
pelecipod Erodona mactroides and the gastropod Heleobia australis. Changes in population density of these species are related to season and depth
because their predators (i.e., highly motile adult crabs and fish) tend to
migrate to deeper subtidal bottoms at the end of fall/winter. Erodona mactroides continues as the largest macrobenthic biomass in shallower subtidal bottoms ( <1.5 m), but occurs together with patches of the burrowing
pelecipod Tagelus plebeius. These shallower areas tend to be covered by
epibenthic microalgae, large quantities of detrital matter, and occasionally
by aggregations of macroalgae, all of which offer protection for small
sedentary peracarid crustaceans (i.e., the tanaid Tanais stanfordi, the
amphipods Melita mangrovi, Ampithoe ramondi, Leptocheirus sp., the isopods Dies fluminensis, Munna peterseni). Furthermore, the abundant food
resources constitute ideal nurseries for juvenile shrimps (Farfantepenaeus
paulensis, Palaemonetes argentinensis ), highly motile decapods ( Callinectes sapidus, Cyrtograpsus angulatus), and estuarine-resident and estuarine-dependent fish. Owing to the dominance of burrowing species, the
intertidal flats appear to be faunistically poor. However, they are an important habitat for peracarids, pelecipods, and infaunal polychaetes (Laeonereis acuta, Nephtys fluviatilis, Heteromastus similis). The juvenile stages of
polychaetes in superficial sediment strata escape intense predation by
decapods and fish in upper intertidal flats where predators have limited
access or by recruitment during winter when predators abandon shallow
waters, while the adults escape predation by moving to deeper sediment
strata where the tube-dwelling peracarid Kalliapseudes schubartii becomes the dominant species (Bemvenuti 1997a, 1998).
12.3.3 Sea Grass Beds
Large areas of the shallow subtidal region( > 1.5 m depth) are colonized by
Ruppia maritima beds. The upper growth of the populations is controlled
by air exposure (>20% per year), while underwater irradiance (mean
monthly Secchi depth 23-78 em) defines the lower growth limit where,
owing to depth-related reproductive strategies, flowers and fruits are
absent (Costa and Seeliger 1989; Seeliger 1997a). The leaves and shoots of
Ruppia provide an important semi-permanent substrate for epiphytic and
epifaunal organisms. Colonization by the diatom Cocconeis placentula
initiates in the spring and, at the end of the summer, older leaf apices are
171
to environmental characteristics. The deep channels (>6 m) have low
macrobenthic abundance and diversity but are an important route for
many migratory organisms. The subtidal soft-bottoms between 1.5- and
5-m depths offer spatial and trophic niches for dense populations of the
pelecipod Erodona mactroides and the gastropod Heleobia australis. Changes in population density of these species are related to season and depth
because their predators (i.e., highly motile adult crabs and fish) tend to
migrate to deeper subtidal bottoms at the end of fall/winter. Erodona mactroides continues as the largest macrobenthic biomass in shallower subtidal bottoms ( <1.5 m), but occurs together with patches of the burrowing
pelecipod Tagelus plebeius. These shallower areas tend to be covered by
epibenthic microalgae, large quantities of detrital matter, and occasionally
by aggregations of macroalgae, all of which offer protection for small
sedentary peracarid crustaceans (i.e., the tanaid Tanais stanfordi, the
amphipods Melita mangrovi, Ampithoe ramondi, Leptocheirus sp., the isopods Dies fluminensis, Munna peterseni). Furthermore, the abundant food
resources constitute ideal nurseries for juvenile shrimps (Farfantepenaeus
paulensis, Palaemonetes argentinensis ), highly motile decapods ( Callinectes sapidus, Cyrtograpsus angulatus), and estuarine-resident and estuarine-dependent fish. Owing to the dominance of burrowing species, the
intertidal flats appear to be faunistically poor. However, they are an important habitat for peracarids, pelecipods, and infaunal polychaetes (Laeonereis acuta, Nephtys fluviatilis, Heteromastus similis). The juvenile stages of
polychaetes in superficial sediment strata escape intense predation by
decapods and fish in upper intertidal flats where predators have limited
access or by recruitment during winter when predators abandon shallow
waters, while the adults escape predation by moving to deeper sediment
strata where the tube-dwelling peracarid Kalliapseudes schubartii becomes the dominant species (Bemvenuti 1997a, 1998).
12.3.3 Sea Grass Beds
Large areas of the shallow subtidal region( > 1.5 m depth) are colonized by
Ruppia maritima beds. The upper growth of the populations is controlled
by air exposure (>20% per year), while underwater irradiance (mean
monthly Secchi depth 23-78 em) defines the lower growth limit where,
owing to depth-related reproductive strategies, flowers and fruits are
absent (Costa and Seeliger 1989; Seeliger 1997a). The leaves and shoots of
Ruppia provide an important semi-permanent substrate for epiphytic and
epifaunal organisms. Colonization by the diatom Cocconeis placentula
initiates in the spring and, at the end of the summer, older leaf apices are