Peter L. McCall

Effect of deposit feeders on migration of137Cs in lake sediments

Abstract Illite clay particles with adsorbed 137 Cs were added as a submillimeter layer to the surface of silt-clay sediments contained in rectangular Plexiglas cells stored in a temperature-regulated aquarium, in order to trace the effect of the oligochaete, Tubifex tubifex , and the amphipod, Pontoporeia hoyi , on mass redistribution near the sediment-water interface. A well-collimated NaI gamma detector scanned each sediment column (∼10 cm deep) at daily or weekly intervals for six months, depicting the time evolution of radioactivity with and without added benthos. In a cell with tubificids (∼5 × 10 4 m −2 ), which feed below 3 cm and defecate on surface sediments, the labeled layer was buried at a rate of 0.052 ± 0.007 cm/day (20°C). When labeled particles entered the feeding zone, 137 Cs reappeared in surface sediments creating a bimodal activity profile. In time, the activity tended toward a uniform distribution over the upper 6 cm, decreasing exponentially below to undetectable levels by 9 cm. In a cell with amphipods (∼1.6 × 10 4 m −2 ) uniform activity developed rapidly (∼17 days) down to a well-defined depth (1.5 cm). The mixing of sediments by Pontoporeia is described by a simple quantitative model of eddy diffusive mixing of sediment solids. The value of the diffusion coefficient, 4.4 cm 2 /yr (7°C) was computed from a least squares fit of theoretical to observed profile broadening over time. In a cell without benthos, small but measurable migration of 137 Cs indicated an effective molecular diffusion coefficient of 0.02 cm 2 /yr.

The Effects of Benthos on Physical Properties of Freshwater Sediments

In this chapter we review the effects of freshwater benthos on the physical (as opposed to chemical) properties of the bottom. Specifically, we will focus our discussion on the effects of macrobenthos (adult length >1 mm) on fine-grained bottoms (sediments that contain approximately 50% by weight silt—clay-sized particles) of lakes and slow-flowing rivers. There are three reasons for this approach. The first is that there is simply too little known about freshwater meio- and microbenthos to merit a review of their effects on sediment properties. More importantly, the macrobenthos are probably the most potent modifiers of sediment properties by virtue of their size relative to sediment grains, their population density, their ability to move through a relatively large volume of sediment, and their feeding and respiratory habits. Finally, we have restricted ourselves to fine-grained bottoms because these comprise the bulk of freshwater lake sediments and because the structure of this sediment is more easily altered than is that of sediments with larger grain size. Some information from slow-flowing river bottoms is included with lakes because their sediments and faunas are very much alike (Hynes, 1970).

137Cs and 210Pb transport and geochronologies in urbanized reservoirs with rapidly increasing sedimentation rates

Sedimentation rates have been measured in three reservoirs in northeastern Ohio, U.S.A., by means of 137Cs and 210Pb geochronologies, volumetric surveys and varve counting. These various methods, while only partially overlapping for each reservoir, show dramatic (three-fold) increases in rates of sediment accumulation in each system between about 1940 and 1977. Mass sedimentation rates are very nearly proportional to the size of the population in the region and possess a doubling time of roughly 19 yr. In these systems with changing sedimentation rates, the preferred model for use with 210Pb geochronologies is one which assumes a constant activity of material added to surface sediments. In systems possessing large watershed)/(reservoir area) ratios, increasing erosion is evidently accompanied by a proportionate increase in the erosion of excess 210Pb. High near-surface activities of 137Cs are due to system integration effects with time constants the order of 10 yr. to a few decades. Total accumulation of fallout 137Cs and excess 210Pb far exceed direct atmospheric loadings, thus indicating the importance of watershed contributions and implying annual retention of the radionuclides in the reservoirs of between ∼15% and ∼80%. In Lake Rockwell, sedimentary fluxes of Zn, Pb and Cu have increased with time. The flux of Cu in particular has increased very markedly since 1970, and concentrations are high in surface materials due in large part to addition of CuSO4, an algicide, to the water. Because of increased rates of sedimentation, the remaining useful life of Lake Rockwell has decreased from 203 to 67 yr., while the remaining useful life of Mayfair Lake is now less than 5 yr.

Biological Redistribution of Lake Sediments by Tubificid Oligochaetes: Branchiura sowerbyi and Limnodrilus hoffmeisteri/Tubifex tubifex

Mechanisms and rates of sediment mixing by the largest oligochaete in Lake Erie, Branchiura sowerbyi, have been quantitatively investigated using a multiple 137Cs tracer layer microcosm technique and compared with a mixture of the dominant tubificids, Limnodrilus hoffmeisteri + Tubifex tubifex. These worms feed head down in the sediment (up to 20 cm for B. sowerbyi and up to 10 cm for L. hoffmeisteri/T. tubifex) on organic-rich particles and deposit fecal pellets at the sediment-water interface (conveyor-belt feeding). Obliteration of tracer layers by these worms was attributed to mixing by both diffusive- and feeding-style (advective) processes. The downward velocities were 2.87 to 3.66 cm/d/100,000 indiv/m2 for the cells with B. sowerbyi (∼10 cm body length, 13 mg body mass) and 0.33 to 0.49 cm/d/100,000 indiv/m2 for the cells with L. hoffmeisteri / T. tubifex (∼5 cm body length, 1 mg body mass). These downward velocities correspond to sediment fluxes across the sediment-water interface of 66.4 to 86.4 g dry sediment/indiv/m2/yr in cells with B. sowerbyi and 5.91 to 9.09 g dry sediment/indiv/m2/yr in the cells with L. hoffmeisteri / T. tubifex). The differences between species was due to differences in biomass, with recycling rates of 5.11 to 6.65 and 5.91 to 9.09 g dry sediment/mg biomass/m2/yr for B. sowerbyi and L. hoffmeisteri/T. tubifex, respectively. Similarly, biomass normalized downward velocities were 806 to 1,028 cm/yr/kg biomass/m2 and 1,205 to 1,789 cm/yr/kg biomass/m2 for B. sowerbyi and L. hoffmeisteri/T. tubifex, respectively. Both B. sowerbyi and L. hoffmeisteri / T. tubifex feed selectively on organic-rich fine-grained particles and showed an increase in particle selectivity with an increase in population density. The particle selectivity factor values ranged from 1.0 to 2.5. Food competition at a higher population density might force these organisms to selectively feed on a smaller size range of sediments. The maximum feeding rate for B. sowerbyi (4,000 to 8,000 indiv/m2) ranged from 9.10 to 13.9 per yr at depths between 11.7 and 13.6 cm while the maximum biodiffusion coefficient, Db, ranged from 0.78 to 1.02 cm2/yr at depths between 1.6 and 2.3 cm. The maximum feeding rate for L. hoffmeisteri / T. tubifex (20,000 to 40,000 indiv/m2) was 8.13 to 13.1 per yr at depths between 5.21 and 5.27 cm and Db ranged from 0.20 to 0.72 cm2/yr at depths between 0.87 and 2.0 cm.

Sediment Mixing by Lampsilis Radiata Siliquoidea (Mollusca) from Western Lake Erie

The sediment reworking activities of an abundant Lake Erie unionid bivalve, Lampsilis radiata siliquoidea, have been studied by field observations and laboratory experiments. Unionid burrowing in laboratory microcosms increased sediment water content 10-20%, decreased water content variability, homogenized sedimentary structures, and increased tenfold the volume of oxidized sediment over that in microcosms with no unionids. Incomplete mixing of sediment took place to a depth of 10 cm during burrowing by L. r. siliquoidea. Burrowing by other unionids may extend the depth of maximum reworking to 20 cm. Unionid burrowing, feeding, and respiratory activities may alter the profiles of various elements and radionuclides associated with sediment particles and alter the location and intensity of microbial activity in sediments.

Kinetics of nutrient and metal release from decomposing lake sediments

Abstract Rates of anaerobic decomposition of Lake Erie sediments were determined for seven depth intervals at three temperatures. Sealed sediment sections were incubated under anoxic conditions and the interstitial waters were serially sampled over a period of approximately 200 days. Concentration increases of bicarbonate, phosphate, ammonium, Ca, Mg, Fe and Mn in pore water within any given depth interval followed zero order kinetics over the sampling period and exhibited Arrhenius temperature dependency. Rates of release to the pore waters were proportional to the concentrations in the solid phases, indicating first order kinetics overall. The rates and temperature dependencies of these fermentation reactions were only slightly less than those reported from sediments undergoing sulfate reduction. The observed release rates decreased exponentially with depth in the sediment due to a corresponding decrease in the amount of metabolizable organic matter and acid hydrolyzable mineral phases. A stoichiometric model was constructed utilizing the observed release rates and assumed chemical reactions to predict the stoichiometry of the decomposing organic matter and the nature of the hydrogen buffer. The modeling indicates that 60% of the observed bicarbonate release is the direct result of organic decomposition, that 20% of the release is from the dissolution of calcium carbonate mineral phases, and that the remaining 20% of the release is from the dissolution of magnesium, iron und manganese carbonate mineral phases. Kinetic modeling of the observed production rates accurately predicts the vertical profiles of Ca, Mg, Fe and Mn, but cannot quantitatively account for all the concentration differences of the nutrient elements C, N and P. This implies that in addition to decomposition, increased depositional flux also accounts for the significant changes in concentrations of the nutrient elements in the near surface sediments.

SPATIAL-TEMPORAL DISTRIBUTIONS OF LONG ISLAND SOUND INFAUNA: THE ROLE OF BOTTOM DISTURBANCE IN A NEARSHORE MARINE HABITAT

Abstract: Field experiments demonstrated a characteristic response of Long Island Sound infauna to substratum disturbance. Relatively opportunistic and equilibrium adaptive strategies were discerned. Three grab sample surveys were made during 1971–1973 to test the hypothesis that benthos spatial and temporal distributions result from differential adaptation to bottom disturbance. Opportunistic species were patchily distributed in space and time; equilibrium species were evenly distributed. Opportunist-dominated stations were found on all bottom types, but in depths of less than twenty meters. Analyses of variance and canonical variate analyses of benthos samples showed that while faunal differences among bottom types do exist, depth-associated differences were most pronounced. Wave hindcasts and observation of storm effects showed that disturbance frequency and intensity were also depth-dependent. The distribution and effects of benthos predators remain unknown. Eighty-five percent of the bivalve species sampled decreased in abundance from 1972 to 1973, while only twenty percent of the polychaetes declined. Three fourths of those species possessing lecithotrophic larvae or brood protection were unaffected or increased in abundance between 1972 and 1973; only 30–40% of the species with long-lived planktotrophic larvae were unaffected. While many benthos distribution patterns appear to result from differential adaptation to bottom disturbance, others are most clearly related to plankton phenomena.

The Effects of Size-Selective Feeding by Oligochaetes on the Physical Properties of River Sediments

ABSTRACT Tubificid oligochaetes selectively ingest silt- to clay-sized particles at depth within the substratum, transport them vertically upward through their gut, and deposit them as feces at the sediment-water interface. These activities form three distinct sedimentary layers. The sediment-water interface becomes covered with sand-sized fecal pellets. A silt-clay layer forms directly below this. The third layer is a sandy concentrate that represents the zone of tubificid feeding. The upper, pelletized layer is enriched in water content and organic carbon. The high water content of this layer, its irregular surface, and the low density of the constituent pellets destabilize the sediment surface and increase its erodability. In addition, the coarse-to-fine layered structure of the deposits fo ms distinctive biogenic graded bedding that is a potentially useful indicator of low current velocities and low rates of inorganic sediment accumulation in ancient fluvial environments.

Soft-Bottom Succession and the Fossil Record

It is perhaps ironic that the editors of this book would emphasize in their own contribution that certain patterns of biotic interactions found in modern soft-bottom communities are not likely to be preserved in the fossil record and that the interpretation of some preserved patterns is problematical. But this is our conclusion with respect to successions on soft bottoms. After examining both live and dead shelled faunas of nearshore clastic facies, we also conclude that the areal distribution of fossil species cannot be used to establish with certainty the dominant controls of the distribution of living fauna. But these conclusions are provisional, and more subtle and clever analysis may eventually vitiate our pessimism and better explain the causes for some distributional patterns.

Particle Mixing Rates of Freshwater Bivalves: Anodonta grandis (Unionidae) and Sphaerium striatinum (Pisidiidae)

Abstract Sediment mixing by freshwater suspension feeding bivalves Anodonta grandis (Unionidae) and Sphaerium striatinum (Pisidiidae) was studied by adding illite clay particles with adsorbed I37 Cs as a submillimeter thick layer to the surface of silt clay sediments contained in clear rectangular cells maintained in a temperature regulated aquarium. A Nal gamma detector scanned the sediment column in each cell at 0.2 cm intervals five times over 22 days and recorded changes in I37 Cs activity over time with depth in cells containing 3 A. grandis , 4 S. striatinum , and a control cell containing no bivalves. Sediment mixing by these organisms was diffusional. The diffusion coefficient in the control cell was 0.02 cm 2 /yr, consistent with molecular diffusion of 137 Cs tracer. Whole cell biodiffusion coefficients (D b )for A. grandis and S. striatinum were 0.81–2.11 cm 2 /yr and 0.53 cm 2 /yr, respectively. Adjusting to equal population densities, the 11-27× higher sediment mixing rate of A. grandis was likely due primarily to its larger size. When D b for similar sized organisms was compared, S. striatinum was found to mix sediments at about the same rate as the marine bivalve Nucula proxima but at a 5× lower rate than the freshwater amphipod Diporeia sp. A. grandis mixes sediments 5–14× more slowly than the similar sized conveyor belt deposit feeding marine bivalve, Yoldia limatula .. While deposit feeding organisms are the dominant sediment mixers in the Great Lakes, suspension feeding bivalves can be locally significant.