Conclusions

1. Values are presented fo r sorption o f the pesticides simazine, atrazine, propazine, desmetryn, prometryn, terbutryn, fenitrothion, malathion, cyanazine and parathion onto blackfly larval silk and the pedal mucus o f two freshwater snails.

2. Pesticide contamination studies are traditionally lim ited largely to materials (soils and sediments) that are inappropriate (in isolation) to environmental studies, in that these materials do not exist in isolation, but are often coated with organic material.

3. Organic materials produced by invertebrates, are a rich food source, and are an important, yet neglected, class o f materials in terms o f sorption studies and fo r models relating to pesticide contamination and retention in the environment.

4. Organic materials produced by invertebrates have a high a ffin ity fo r pesticides, capable o f sorbing such compounds to a far greater degree than soils or sediments alone.

5. No statistical differences were found between the sorption o f pesticide compounds. Coupled w ith the high sorption o f hydrophilic as well as hydrophobic compounds, this suggests that the mechanism o f pesticide sorption onto mucus and silk is determined by the nature o f the organic materials, rather than by the chemical nature, atomic weight or structure o f the pesticide compounds. Characteristics such as pesticide atomic weight or structure cannot, however, be assumed to have no influence on their sorption on to materials w ith in the water column.

6. By sorbing onto organic materials o f such high food value, pesticides are accessible to the food chain, leading to im plications o f bio-accumulation.

153

Table 4.12

D efinition o f parameters used in this study.

The distribution coefficient fo r mucus produced by a known wet weight o f snails:

=

(A m o u n t o f p e s t i c i d e s o r b e d o n t o m ucu s) |ig

(m a s s o f w e t s n a i l s t h a t p r o d u c e d t h e m ucu s) g ° \ ( c o n c e n t r a t i o n o f p e s t i c i d e e s o l u t i o n a f t e r 24 h e q u a l i h r a t i o n )

Hence, K„ has units of:

ml {wet w ei g ht o f s n a i l )

Using the conversion factor o f 3.6 x 10^ g (mucus) g * (snail) d

(A m o u n t o f p e s t i c i d e s o r b e d o n t o m ucus)

^ I ___________________5_________________(m ass o f m u c u s ) g________________[________________

[ c o n c e n t r a t i o n o f p e s t i c i d e e s o l u t i o n a f t e r 24 h e q u a l i b r a t i o n )

So that has units of:

= ml gr-i {mucus)

Sim ilarly, the distribution coefficient fo r silk is defined as:

K. = (A m o u n t o f p e s t i c i d e s o r b e d o n t o s i l k ) \ig ________________(m ass o f s i l k ) g _________ \ ( c o n c e n t r a t i o n o f p e s t i c i d e € s o l u t i o n a f t e r 24 h e q u a l i b r a t i o n ) jig m l'^^ giving in units o f : = m l ( s i l k )

Chapter 5 Sediment stability

5.1 Introduction

In looking at invertebrate distribution in shallow lakes in relation to sediment stability, Moss and Tim m s (1989) found greater invertebrate colonisation in stable sediments. By offering such stable areas to invertebrates (w ith sediment, free o f macroinvertebrates, allowed to settle in plastic bowls, covered in protective netting and buried such that it was level w ith the surrounding sediment), they concluded that structure o f the sediment, rather than food content, was the prim ary attraction fo r oligochaetes and chironomids. It is possible that organisms such as these, in secreting mucus and silk, w ill also contribute to the stability o f sediments. Moss and Timm s did not, however, venture any hypotheses on the means by which sediments in bowls become more stable in the first place. W ork on microaggregates o f picoplankton by K lu t and Stockner (1991) may provide some clue as to how this process, or rather cycle, begins. They describe small aggregates o f planktonic cells that seasonally form in order to reduce the risk o f herbivory. O f specific interest to my study is the observation that, as nutrient availability declines seasonally, greater quantities o f unspecified "non diffusible cell exudates" are produced. It is conceivable that such exudates are polysaccharide-based, and that, fo llo w in g senescence o f such microaggregates, they m ight sink. In so doing, they w ill settle upon and blanket benthic sediments, so producing the stable areas described by Moss and Timm s (1989), that m ight favour invertebrate colonisation.

It is only recently however, that such material as TEP and microaggregates (albeit in a different form , ie colloidal organic carbon aggregates (Kepkay, 1994); estuarine TEP (Syvitski et al., 1995); suspended solid's aggregates, (M aldiney and Mouchel, 1995); CEP (W otton, 1996); particle aggregates (Gregory, 1997)), have been noted to exist in lotie systems. Carlough (1994) looked at particulate amorphous seston in a blackwater river, composed o f "clay, organic debris and microorganisms w ith in a matrix o f mucopolysaccharide fib rils". Exam ining b io film on wood in the same river. Couch and Meyer (1992) noted that m icrobial extracellular polysaccharide may be

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assimilated by detritivores. However, it was W otton (1996) who first made the connection that the recently categorised TEP also existed as a sim ilar material (as yet unnamed), in rivers as w ell as oceans. In reviewing colloids, bubbles and aggregates w ith regard to their role in suspension feeding, he noted that colloidal exopolymer particles (CEP) act as a vehicle fo r the aggregation o f dissolved organic matter. Others were certainly aware o f such suspended aggregates and M aldiney and Mouchel (1995) developed a new means o f studying them in situ. W h ilst they noted that aggregation accounts fo r the "biogeochemical fate" (via sedimentation) o f a proportion o f certain contaminants via sticky polysaccharides, their main aim was to observe suspended particles rather than speculate on the effect, or source, o f such floes. It was thought that most aggregates would settle on to sediment, where, due to greater turbulence, shear stresses would erode floes, so depositing pollutants onto sediments. It is not unreasonable to assume that remnants o f sticky polysaccharide floes w ill also settle upon sediments, possibly sealing in, or encapsulating whatever particles / pollutants they have transported. This assumption is particularly plausible when one considers the action o f macroinvertebrates such as snails which coat sediments in trails o f polysaccharide. A d d itio n a lly, tube builders utilise particulate matter, w hile burrowing organisms drag organic material below the surface o f sediments (Dudgeon,

1990).

5.1.1 The role o f macroinvertebrates in sediment stabilisation and

disturbance

A ll the studies examining sediment consolidation cited in Chapters One and Five, have looked largely at bacterial mucopolysaccharide. O nly Moss and Timm s (1989) hinted that something other than bacteria may be responsible for the in itia l prim ary cohesion o f sediments that bacteria subsequently stabilise. Scheffer et al., (1993) suggested that lake sediments undergo a cycle, such that lake waters are alternately turbid and clear. W hilst this is probably due in part, to algal or bacterial blooms, there is a gap in the theory as to what provides an initial foothold fo r the stabilisation o f sediments in lakes. Macroinvertebrates may provide the answer via the bioturbation they cause and possible stability they lend to benthic sediments. Blauchard et a i, (1997) used snails

as agents o f bioturbation, w hile burrowing organisms are known to stabilise the walls o f their burrows with polysaccharide (de Vaugelas and Buscail, 1990). Furthermore, Chapter T w o illustrates the sustained attachment o f sediment particles to STM. There is then a gap in the literature concerning the contribution made to sediment stability by freshwater macroinvertebrates, or indeed, macroinvertebrates o f any aquatic environment.

The fo llo w in g experiments are an attempt to investigate whether macroinvertebrates, (in this case, river snails) promote sediment stability. W hilst tub ificid s or chironomids may w ork sediment, coating discrete packages with organic material, snails coat greater areas w ith polysaccharide simply through greater d aily m obility. Such areas may then hold particles that impact upon a surface, or may serve to bind lose sediment and fine particles long enough to allow fo r bacteria to produce further quantities o f polysaccharide in terstitially. Gastropods are known to actively farm the bacteria that impinge upon their mucus trails (Connor and Quinn 1984), and I have shown that snail trails persist over a number o f days (Chapter Tw o). Each o f the fo u r experiments was an attempt to quantify the cohesive properties o f polysaccharide, or other organic material, w ith in a lotie system. Organic material, bacterial mucopolysaccharide and snail trail mucopolysaccharide are the three basic ingredients under trial, in various combinations.

5.1.2 Experimental objectives

The main objective was to design, construct and calibrate a "lo w cost - low tech" procedure that would measure accurately, the cohesion o f sediments. Using a number o f different sediment preparation procedures, additional objectives were the quantification o f the degree o f cohesion offered to sediment by:

1) Undefined natural organic, and inorganic, fractions (that provide stability to sediments);

2) Snail mucus w ith undefined natural organic, and inorganic, fractions;

3) Snail mucus alone;

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5) Bacterial EPS, algal exudates and other materials that exist, or are swept up, in the water column rather than being bound to sediments, and;

6) The impact o f macroinvertebrate sediment dwellers;

5.2 Materials and Methods

Preparation o f dried sediment

Wet sediment was removed from the Botany Pond, fed by the M ill Stream leat o f the River Frome, East Stoke, Dorset (NGR: SY 876 868). M aterial was collected from the surface o f the pond sediment, using a pond net, up to a depth o f 20 cm. Sediment was allowed to settle, was decanted, then sieved as a slurry through a 2.0 mm sieve to remove coarse matter. Sediment was further allowed to settle overnight, before being decanted, placed in metal baking trays and dried at 60®C. Pieces o f dried sediment were ground in a mortar and pestle and sieved, the fraction between 250 |im and 500 |im being retained. This size fraction was placed in ceramic crucibles and burned, using methanol, to remove excess organic material (an attempt to reduce noxious smoke emissions w ith in the laboratory). The remaining sediment was ashed in a m uffle furnace at 550°C fo r at least 12 hours, to remove all organic material, leaving only dry mineral sediment (Figure 5.1).

Fresh Sediment

Wet sediment was collected from the Botany Pond as above, and wet sieved, retaining the fraction between 250 |Xm and 500 jlm (Figure 5.1) and therefore retaining all organic matter o f this size.

Sediment sampled from top 20 cm

oily, sieved at 2 mm and allowed to settle

Sediment prqjaraticMi D 1

Sediment preparation D 2

Allowed to settle, water decanted off, cooked for 48 hours at ^ 60®C to remove pore water. Sieved (250-500 pm), flamed

and ashed at 550°C for 12 hours to ranove all organic material. Sediment preparation B 1 Sediment preparation A 1 ▼ Sedimmt preparation A 2 ▼

Tanks ofwel unwashed sediment, covered with riva wata and left for 7 days.

/

Tanks of dried organic free sediment, one left in riva wata with snails for 7 days, the otha left ovalaid by an antibiotic solution.

Glass tank of dried prepared sedimait and snails.

1

Schematic diagram to illustrate the preparation o f each o f the four sediment types with associated controls.

Mesh bag of sediment, suspended in flowing riva water

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