Tris Wooldridge

Sand Beach Energetics: An Ecosystem Approach Towards a High Energy Interface

General results of a study of energetics on open sandy beaches in South Africa are presented. These sand beaches are considered to interact with adjacent terrestrial environments via the sand dune system and with the sea via the surf zone. A food web is given for the macrofauna showing all known interactions from the supply of food material to the beach, mainly from the sea, to the removal of the macrofauna by birds and fishes. An energy circuit diagram is presented quantifying the main energy flows through this system of filter feeders and scavengers. The interstitial biota of these beaches is considered separate from the macrofauna and consists of bacteria, protozoa and meiofauna feeding on dissolved and particulate organics flushed into the beaches by wave and tide action. Interstitial energy flow and nutrient cycling rates are quantified in an energy circuit diagram. It is suggested that nutrients regenerated by this latter system in the intertidal and surf zone, as well as by the activities of the macrofauna, have sufficient residence times in the surf zone to cause blooms of surf zone phytoplankton which in turn are the main food for the intertidal filter feeders. In this respect the beach and surf zone may represent a more closed system than previously thought. A combined energy circuit diagram is given depicting the beach and surf zone as an ecosystem with the surf zone phytoplankton the producers, the macrofauna the consumers and the interstitial fauna the decomposers. Main imports and exports as well as the consequences of this ecosystem approach are discussed.

What limits the distribution of subtidal macrobenthos in permanently open and temporarily open/closed South African estuaries? Salinity vs. sediment particle size

Abstract Thirteen estuaries in the Eastern Cape Province, South Africa, were assigned to one of four types of estuaries (river-dominated permanently open, marine-dominated permanently open, medium-sized temporarily open/closed and small temporarily open/closed), and macrobenthic zonation patterns were compared using multivariate statistics. Three major habitat zones were identified: two relating to substrate (a sand zone and a mud zone), and a third zone characterised by water of low salinity (≤2.2). These zones are inhabited by four major groups of macrobenthos. The distribution of two of these (estuarine endemic sand fauna and estuarine endemic mud fauna) is limited by the nature of the substratum, whereas the distribution of the other two (fauna originating from the marine habitat and oligohaline fauna) is limited by salinity. Estuarine endemics were present in all four types of estuaries. Marine fauna was numerically important only in permanently open systems, while oligohaline fauna was abundant only in the upper reaches of river dominated open systems. Medium-sized temporarily open/closed estuaries lacked both marine and oligohaline forms, but these were present to a limited extent in small temporarily open/closed estuaries. Hence, salinity is not the primary environmental variable determining zonation patterns in Eastern Cape estuaries, and particularly in temporarily open/closed estuaries, it is of minor importance.

A comparison of the macrobenthic faunas of permanently open and temporarily open/closed South African estuaries

Thirteen estuaries in the Eastern Cape Province, South Africa, were broadly categorised according to size and salinity distribution and were assigned to one of the following categories: permanently open estuaries having a strong salinity gradient between mouth and upper estuary, freshwater-deprived permanently open estuaries, medium-sized temporarily open/closed estuaries, and small, temporarily open/closed estuaries. The macrobenthos collected during surveys was then compared in terms of the following parameters: species composition, salinity, sediment mud content, density of macrobenthic animals, Hills N0 (species richness), and Hills N1 (diversity). Mud content was found to be the most important environmental variable responsible for biotic patterns found, and sites were consequently assigned to either a sand zone fauna, or a mud zone fauna. Both types of fauna are present in all estuaries sampled, with upper sites of river dominated estuaries having an additional oligohaline fauna, and freshwater-deprived estuaries providing habitat for many marine species. Small, temporarily open/closed estuaries have the highest macrobenthic density, whereas N0 and N1 are highest in freshwater-deprived permanently open systems. River-dominated permanently open estuaries tend to have lower macrobenthic densities, species richness, and diversities compared to estuaries in the other categories. No seasonal differences in these ecological indices were found within any of the estuarine categories.

Origin and trophic importance of detritus - Evidence from stable isotopes in the benthos of a small, temperate estuary

Ratios of 13C/12C and 15N/14N were measured in dissolved inorganic carbon (DIC), marginal vegetation, benthic macrodetritus (diameter > 1 mm) and selected invertebrate consumers in the Gamtoos estuary, South Africa to: (1) trace the provenance of benthic detrital deposits, and (2) determine the extent to which three abundant species of macroinvertebrates utilise this resource. DIC was strongly depleted in 13C with average δ13C values (−9.5±0.5‰) being typical of limnetic waters. Benthic detrital particles (δ13C−24.1±0.3‰) originated mainly from marginal vegetation (δ13C−25.7±0.3‰), but their slightly elevated carbon ratio suggests additional input from 13C-rich sources-possibly C4 plants cultivated on the floodplain. Populations of the fossorial ghost shrimp Callianassa kraussi, the bentho-pelagic amphipod Grandidierella lignorum and the epifaunal crab Hymenosoma robiculare together account for 96% of total benthic biomass in the upper regions of this estuary. Marked differences in trophic niches were evident among these three consumer species. Ghost shrimp (δ13C −32.5±0.3‰) foraged by filter-feeding on fine suspended particulate organic matter (δ13C−31.2±0.5‰). Amphipods (δ13C−28.0±0.6‰) utilised some benthic detritus but fed mainly on suspended material. Only the relatively rare crabs (δ13C−23.8±1.5‰) appeared to utilise benthic detrital particles to any significant extent. In the benthic consumer community of the upper Gamtoos estuary, suspension feeders make up 98% of biomass and thus clearly dominate over deposit feeders. This can be traced to the low contribution of higher plants (c. 13%) to overall carbon production, and detritus originating from macrophytes is consequently relatively unimportant in supporting invertebrate secondary production in this particular system.

How sieve mesh size affects sample estimates of estuarine benthic macrofauna

The way mesh size influences the reliability of density and biomass estimates in macrofauna sampling was investigated for a benthic community in the shallow, upper reaches of a small, temperate estuary. Macrofauna was separated from the sediment with sieves of 0.25, 0.5 and 1.0 mm mesh aperture size and the performance of each gear type in terms of retention efficiency evaluated. The 0.25 mm sieve sampled all macrofauna adequately, but only 55% of all individuals were, on average, retained by the 0.5 mm mesh and a mere 8% by the 1.0 mm mesh. Mean retention efficiency for total biomass was 86% for the 0.5 mm and 49% for the 1.0 mm screen, respectively. Undersampling by bigger mesh sizes was most severe for juveniles and small species, resulting in biased interpretation of apparent community structure. Because population densities obtained by the two bigger screens were not only markedly less accurate (i.e., biased lower counts), but also less precise (i.e., higher standard errors), the use of 0.5 mm and 1.0 mm mesh sizes significantly increased replication levels. Consequently, for this specific habitat and community, sampling schemes which employ a 1.0 mm mesh were considerably more expensive than designs using finer screens. No single sampling design will perform optimally in each permutation of habitat and community, but our data emphasise the need for the stringent assessment of sampling errors and caution against the uncritical use of mesh sizes greater than 0.25 mm in estimating population densities of estuarine macrobenthos.

Population dynamics and estimates of production for the calanoid copepod Pseudodiaptomus hessei in a warm temperate estuary

Abstract The copepod, Pseudodiaptomus hessie, was studied over 1 year in the Sundays River estuary. Maximum abundance occurred during summer in the upper half of the estuary. Nauplii and copepodid stages attained peak abundance higher up in the estuary compared with adults. Development time and body size were inversely related to temperature and varied between instars. Females took longer to reach maturity than males. Reproduction was continuous. Production estimates integrated over 1 year varied from 0·3 g dry mass m−3 at the estuary mouth to 4·3 g dry mass m−3 in the upper estuary. Annual P/B ratios varied between 78·5 and 100·2 for the same regions. High production estimates can be linked to high summer temperatures.

Axial Zonation Patterns of Subtidal Macrozoobenthos in the Gamtoos Estuary, South Africa

The distribution of macroinfauna was quantified in subtidal, soft-bottom habitats, extending from the estuarine mouth to the tidal head of the Gamtoos—a small, shallow, temperate estuary situated on the south coast of South Africa. Sampling covered the full salinity gradient from fresh to marine waters, and all sediment types from marine sands to fluvial silts. A total of 35 taxa was recorded, of which 22 occurred throughout the year. Species richness and diversity declined from the seawater-dominated mouth region toward the fresh water section at the tidal head of the estuary. Sediment type generally bore no clear relation to biotic diversity. A marked drop in salinity between winter and summer sample series (Δ 0.2‰ to 24‰) coincided with a reduction of mean macrofaunal density by 70%, a more seaward relocation, and a compression of axial ranges of most taxa. Numerical classification and ordination of faunistically similar regions and of co-occurring species delineated four habitat zones along the longitudinal axis of the estuary which harbour four distinct macrofaunal assemblages: 1) A tidal inlet area with salinities close to seawater; clean, coarse, marine sands, rich in CaCO3 harbour a stenohaline fauna normally found on adjacent, marine sandy beaches. 2) In the lower reaches, where fine, fluvial silts of high organic content prevail, euryhaline polychaetes dominate the macrozoobenthic community; bottom salinities in this zone seldom dropped below 25‰ 3) The middle reaches, characterized by oligohaline- to polyhaline waters, stretch over sandy sediments of intermediate carbonate, silt, and organic fractions; the fauna comprises typical estuarine forms, which occurred throughout most of the estuary except at its seaward and landward limits. 4) The upper reaches encompass the limnetic waters near the tidal head of the estuary with sediments in this zone being composed mostly of coarse, clean sands, low in CaCO3; the macrobenthos in this region is dominated by taxa of freshwater origin, which generally do not penetrate seaward beyond the oligohaline waters, and by exceptionally euryhaline estuarine species. Salinity appears as the main factor in controlling faunal assemblages at both extremes of the estuarine gradient (i.e., tidal inlet and head), whereas sediment type delineates between communities in the mesohaline to polyhaline reaches. Axial (i.e., from tidal inlet to tidal head of the estuary) zonation patterns of macroinfauna broadly matched those of mesozooplankton and fishes, supporting the notion of a general structure underlying species distribution patterns in the Gamtoos estuary.

Feeding of two mysid species on plankton in a temperate South African estuary

Two sympatric mysid species, Mesopodopsis wooldridgei and Rhopalophthalmus terranatalis, dominate mesozooplankton biomass in the Sundays River estuary, South Africa. This study aimed to investigate the food and feeding rates of the two mysids on naturally occurring phytoplankton and microzooplankton assemblages, and on copepod instars, using laboratory based feeding and predation experiments. Maximum clearance rates calculated for different size classes of M. wooldridgei on microplankton were on average 34.2, 15.8 and 18.6 ml ·. ind−1 ·. h−1 for adults, immatures and juveniles respectively. For the same R. terranatalis size classes clearance rates were 8.5, 13.0 and 13.5 ml ·. ind1 ·. h−1 respectively. Thus, all size classes of the smaller M. wooldridgei had higher maximum clearance rates compared to any of the stages of the larger R. terranatalis, indicating that the former species fed more successfully on phyto- and microzooplankton and that these prey assemblages probably constitute the lower limit of the food particle size range consumed by the latter species. R. terranatalis is more successful in preying on larger plankton such as adult copepods and juvenile mysids. Microzooplankton was generally more important in mysid diet than phytoplankton. Apparent selection for microzooplankton species was probably due to an inability to feed efficiently on the smaller phytoplankton cells. Copepod nauplii were readily consumed by all mysid size classes. Predation rates, in terms of weight specific carbon ingestion, increased linearly with copepod instar prey concentration.

Distribution, population dynamics and estimates of production for the estuarine mysid, Rhopalophthalmus terranatalis

The biology of the mysid shrimp Rhopalophthalmus terranatalis is described from a warm temperate estuary in southern Africa. Intensive quantitative field sampling continued over 21 months while laboratory culture provided detailed information for production estimates. Three generations are produced annually and individual females produce multiple broods. The overwintering generation has a lifespan of 9–10 months and is characterized by low population density, but individuals are relatively large with females producing large broods. The summer generation survives for 6 months. Population density is high (maximum may exceed 4000 m−3 of water) but individuals attain a smaller size and produce fewer progeny. The spring generation has characteristics which may generally be described as intermediate between the other two generations. Daily production values of 2·69, 6·93 and 13·97 mg m−3 were calculated for the overwintering, spring and summer generations, respectively. Daily PB coefficients varied between 0·020 and 0·026. The PB ratio for the overwintering generation was 5·86; for the spring generation, 3·06 and for the summer generation 4·74. The annual PB ratio for the whole population was 7·85. Comparative values using a second method of population production are provided. The biology of R. terranatalis allows for rapid recovery of the population in a region where catastrophic aperiodic fresh-water flooding occurs.

The effects of mass, temperature and molting on the respiration of Macropetasma africanus balss (Decapoda: Penaeoidea)

Abstract 1. 1. The effects of mass, temperature and molting on the respiration of the surf zone penaeid Macropetasma africanus were examined using a flow through system. 2. 2. Respiration (ascribed to routine metabolism) is described by a power curve: R = aM b ( R = μ gO 2 hr −1 , M = mg dry mass), which gives values of a = 1.806, 2.164, 2.964, 3.964, 3.599 and 4.592, and b = 0.799, 0.880, 0.857, 0.840 and 0.959 at 15, 17, 20, 22 and 25°C, respectively. 3. 3. The expression R = 0.0115 T 1.858 M 0.847 provides a reasonable prediction of respiration as a combined function of prawn size and temperature ( T ,°C). 4. 4. In response to approaching ecdysis the oxygen consumption remains at an elevated level until 5 hr before molting followed by a rapid increase reaching a peak at molting. Oxygen consumption decreases rapidly after molting.