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African Journal of Environmental Economics and Management ISSN 2375-0707 Vol. 10 (4), pp. 001-005, April, 
2022. Available online at www.internationalscholarsjournals.org © International Scholars Journals 

 

Author(s) retain the copyright of this article. 
 
 
 

Review 

 

Resuspension of sediment as a method for 
managing shallow eutrophic lagoons 

 
Mauro Lenzi 

 
Lagoon Ecology and Aquaculture Laboratory (LEALab), OPLrl Company, Italy. 

E-mail: lealab1@gmail.com. Tel: +0564860288. Fax: +0564867572. 
 

Accepted 21 August, 2021 
 

The article sets out to demonstrate that resuspension of sediment could be used to counteract the 
consequences of eutrophication and to manage degraded coastal lagoons subject to frequent periodic 
dystrophic processes. This is done by a general review of the literature on resuspension of sediment, 
focusing on its effects in shallow eutrophic lagoons and the results of specific research recently 
conducted by the author. 

 
Key words: Lagoon, eutrophication, sediment resuspension, lagoon biogeochemistry, lagoon management. 

 
 
INTRODUCTION 

 
Bottom disturbance causing resuspension of sediment in 
the water column is usually considered a negative 
phenomenon. Resuspension of normally steady soft 
sediment, for example by gales, can have a considerable 
impact on the water body by raising organic particles and 
dissolved organic matter towards the surface (Hopkinson, 
1985), activating bacterial oxidative mineralization 
(Fanning et al., 1982) and increasing the remineralisation 
rate (Wainright, 1987; Wainright, 1990). Resuspension 
may also activate transport of nutrients in sediment 
interstitial water and adsorbed on sediment particles. 
Using a simulation model, Wainright and Hopkinson 
(1997) established that resuspension causes release of 
nutrients, an increase in carbonates and increased 
oxygen consumption in the water column. However, there 
are contradictory results and different opinions about the 
effects of resuspension of sediment on the water column. 
In fact, much depends on the nature of the sediments, the 
height of the water column, the quality of organic matter 
present and the environmental conditions under which 
disturbance occurs (Tengeberg et al., 2003; Arnosti and 
Holmer, 2003).  

Resuspension of sediment may lead to oxidation of iron 
to ferric oxides, which adsorb orthophosphates, removing 
them from the water column (Golterman, 1995; 2001). 
Thus the impact of resuspension may differ considerably 
from one place to another, in relation, for example, to the 

 
 
 
 

 
amount of iron available in sediments. According to 
Søndergaard et al. (1992), in Lake Aresø (Denmark), 
where sediments are low in iron, resuspension is the 
main factor determining an increase in orthophosphates. 
On the other hand, in a laboratory study, Sloth et al. 
(1996) found that resuspension multiplied dissolved 
oxygen (DO) consumption by ten with respect to control, 
but caused modest changes in nutrient flow. In laboratory 
experiments on deep water sediments, Koschinsky et al. 
(2001) did not find increases in dissolved organic carbon, 
heavy metals, nutrients or microbiological activity as a 
result of adsorption by resuspended sediment matter. 
Tengeberg et al. (2003) found that natural resuspension 
in a Swedish coastal area in winter reduced carbonates 
and phosphates in the water column and increased DO, 
silicates, nitrites and nitrates, while ammonia remained 
unchanged.  

According to these authors, the effects of resuspension 
can vary in relation to bottom conditions and season. 
According to Blackburn (1997), a resus-pension event 
leads to a sudden increase in nitrogen, by mixing 
interstitial water into the water column, but the same 
amount would be released in any case by diffusion 
gradient over a longer period. Tengberg et al. (2003) 
sustain that more field research is needed to clarify the 
effects of resuspension in different seasons of the year 
and its long-term effects. 



 
 
 

 

REPEATED DISTURBANCE AND RESUSPENSION OF 
SEDIMENT 

 

The effects of occasional resuspension are one case, the 
effects of which may vary in relation to sediment 
conditions, accumulation of organic matter, the quality of 
the latter and water column conditions. The effects of 
disturbance repeated with a relatively high frequency may 
be quite different.  

In laboratory studies, Stahlberg et al. (2006) showed 
that frequent resuspension of sediment increased the 
mineralization rate with respect to undisturbed sediment 
by a factor between 2 and 5. In an experiment conduced 
in a limited area of a shallow lagoon (Orbetello lagoon, 
Italy), Lenzi et al. (2005) found that repeated passages of 
boats that resuspended soft surface sediment increased 
the oxidative status of sediments (increase in redox 
potential - Eh), reducing the organic content, without any 
significant increase in nutrients or oxygen consumption in 
the water column.  

Lenzi et al. (2010) found substantially the same results 
in two large areas (20 ha each) of another coastal lagoon 
(Lake Burano, Italy) when surface sediment was 
repeatedly disturbed with a specially fitted boat. The trend 
of dissolved sulphides in this experiment suggested that 
disturbance interrupted sulphate reduction processes. A 
possible hypothesis may be that an area subject to 
frequent turbulence affecting surface sediment, for 
example every 24-48 h as stated by Stahlberg et al. 
(2006), may undergo an increase in the mineralization 
rate, as sustained by these authors, without showing any 
significant effect on the water column, in line with Lenzi et 
al. (2005; 2010).  

In sediments with oxidative status, orthophosphates 
bind to ferric oxides-hydroxides (Golterman, 1995; 2001), 
carbonates and clays (Dodge et al., 1984; De Jonge and 
Villerius, 1989), becoming increasingly rare in interstitial 
waters and released less into the water column, in other 
words, unavailable to algae. As far as nitrogen 
compounds are concerned, oxidation accelerates 
nitrification and leads to pre-dominance of nitrates over 
the reduced forms, nitrite and ammonium (Revsbech et 
al., 1980). An increase in nitrate concentrations produces 
an increase in denitrification (Herbert and Nedwell, 1990), 
which occurs in anoxic microhabitats (in an oxidative 
milieu) (Fenchel, 1992), with the result that part of the 

sediment nitrogen is lost as N2 or N2O and the eutrophy 

of the system declines (Novicki et al., 1997). Thus, when 
a sedimentary substrate undergoes frequent disturbance, 
oxidation occurs, labile organic matter is reduced, 
orthophosphates are not released and there is partial loss 
of nitrogen. 

 

Eutrophication problem in shallow water lagoons 
 
In marine sediments, including those of lagoons and 
other transition environments, more than 50% of organic 
matter is degraded by bacterial sulphate-reduction 

 
 
 
 

 

processes (sulphate respiration) (Jørgensen, 1983). 
Many coastal lagoons and estuaries become eutrophic 
and produce excessive macroalgal biomass in warm 
months (Morand and Briand, 1996). These conditions 
lead to major build up of organic matter in sediments, 
increasing sulphate-reduction. Breakdown of organic 

matter by this process produces the acidifying gases CO2 

and H2S.  
Dissolved sulphides are toxic and may have 

considerable impact (Hijs et al., 2000). This may act on 
bicarbonate equilibria of interstitial water and water in 
contact with sediment, producing a white precipitate of 

CaCO3 (Deelman, 1975). The sediment becomes more 
acidic and more reduced (very low Eh values), leading to 
build-up of reduced and reducing components. Low redox 
potential and pH lead to production of ammonium (Marty 
et al., 1990) and nitrite by ammonification of organic 
matter, and this too has a toxic effect on fauna (Torres-
Beristain et al., 2006), besides stimulating production of 
nitrophilic algal species. 

 

Natural baffering 

 
Natural buffering hinders this trend. Production of free 

sulphide (H2S, HS
-
, S

2-
) by sulphate-reduction is  

countered by ferrous and ferric ions: H2S is oxidized by ferric 

iron (H2S↑+2Fe
+++

 = S↓ + 2Fe
++

 + 2H
+
) and blocked  

as sulphide by ferrous iron, as ferrous sulphide (H2S↑+Fe
++

 

= FeS↓ + 2H
+
) and then as pyrite (FeS↓ + S↓ 

= FeS2↓) (Berner, 1984; Luther, 1991; Richard and 
Luther, 1997; Theberge and Luther, 1997; Rozan et al., 
2002). The pool of iron available in sediment may be 
sufficient to block dystrophic episodes, but if the quantity 
of organic matter exceeds availability (H2S/Fe>1), then 
H2S enters the water column (Giordani et al., 1996), with 
reducing action (removal of oxygen and anoxia) and toxic 
effects. Reduction of iron and its blockade by H2S 
releases orthophosphates previously bound to ferric 
oxides-hydroxides (Gunnars and Blomqvist, 1997; 
Golterman, 2001; Rozan et al., 2002). Thus ortho-
phosphates are mobilised into the water column, where 
they are available to algae. 
 

 

Macroalgal and microalgal blooms 

 

In coastal lagoons with little water exchange, the anoxia 
established in sediments due to high nutrient load and 
accumulation of organic matter leads to increasing degra-
dation and continual stress for populations in the warm 
season, resulting in selection of opportunistic species and 
in a sudden and drastic change in the phytobenthos 
(Valiela et al., 1997). This is immediately evident for 
rooted plants, development of which is curbed by 
bacterial and chemical conditions in the sediment, by 
epiphytes development, by phytoplankton that excludes 
light and by floating macroalgae that can suffocate 



  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

 
Figure 1. Scheme of biogeochemical events induced by 
resuspension of sediment in eutrophic lagoon environments. 

 

 

seagrass meadows (Den, 1994; Raffaelli et al., 1998). 
High eutrophication favours the change from seagrass to 
seaweed, and if conditions worsen, to oppor-tunistic 
microphytes, which have high turnover (Duarte, 1995). 
Lenzi et al. (2003) observed a macroalgal distribution 
gradient with phosphorophilic macroalgae near a man-
made nutrient source and nitrophilic macro-algae at some 
distance from it. Finally, anoxia and the bacteria 
associated with the sulphur cycle may mobilise mineral 
contaminants, such as mercury in cinnabar as 

methylmercury (CH3Hg
+
) (Wood and Wang, 1983). 

These mechanisms are not yet well known and call for 
further research. 
 

 

CONCLUSIONS: A POSSIBLE TOOL FOR LAGOON 
MANAGEMENT 

 

In the framework of such eutrophic environments with 
little water exchange, frequent disturbance of sediment 
counters persistence of dystrophic conditions. By virtue of 
the geo-chemico-physical effects that it produces, 
disturbance leads to a new selection of bacterial, plant 
and animal populations, in the opposite direction to the 
selection produced by eutrophy and dystrophy: oxidative 
mineralising strains of bacteria increase, fall-out of 
resuspended matter increases sediment bacterial activity 
in the vicinity of the disturbed area (Logan and Kirchman, 
1991), infauna biodiversity increases (Widdicombe and 
Austen, 2001), macroalgae suffer phosphorus limitation 
(Lenzi et al., 2003) and recolonization of the substrate by 
phanerogams that capture nutrients directly through their 
roots is promoted. A simplified scheme of the process is 
shown in Figure 1.  

This result was evident during operations carried out to 
combat severe eutrophication of Orbetello lagoon, which 

 
 

 

in the first years consisted largely of macroalgal removal 
by harvesting boats. Though only 3-6% of the estimated 
maximum macroalgal standing crop was harvested, a 
radical change in vegetation quality and quantity was 
obtained in only two years. Chlorophyta decreased 
sharply, Rhodophyta took their place but developed less, 
and phanerogams which had been completely eliminated 
by the environmental crisis, returned (Lenzi et al., 2003). 
The subsequent experiment with a special boat equipped 
to raise soft sediment in Lake Burano showed a drastic 
reduction in macroalgal populations and an increase in 
phanerogams (with many seedlings establishing directly 
from seed) in disturbed areas (total, 44 ha) but not in 
undisturbed areas (total, 40 ha) (Lenzi et al., 2010).  

Much has still to be done to establish whether the 
answer to our initial question is positive, however the 
results so far are encouraging for eutrophic shallow-water 
environments. Under these conditions, it has little sense 
wondering whether sediment disturbance can somehow 
damage the ecosystem, since it is already severely 
threatened by frequent dystrophic crises. Fish of 
commercial interest are often severely impoverished, 
many microenvironments and typical species are lost and 
biodiversity is generally reduced in these areas. Of 
course it is prudent to check sediments for contaminants, 
as some are best not diffused in the water column (Kim et 
al., 2006; Kalnejais et al., 2007).  

Shallow bottoms make disturbance easy using boats of 
a suitable size or fitted with motors that mix air and water 
and direct a jet towards the bottom, suspending the top 3 
to 4 cm of sediment. It is more complex, but not 
impossible, to disturb the bottoms of lagoons and 
estuaries deeper than 2 m. For example, a trawl for 
catching flatfish and pectinids, known as “rapido” 
(Franceschini et al., 2000), could be modified by 
removing the net and teeth that engage the bottom and 



 
 
 

 

adjusting the skids to avoid direct contact with the bottom. 
Dragged by a boat, this device could be used to disturb 
and resuspend superficial sediment in relatively deep 
water.  

It would be worth analysing whether this management 
criterion gives better results than more conventional 
measures, which are mainly of an engineering nature, 
such as creation of underwater channels where sediment 
collects, cement banks, pumping stations to admit sea 
water and flow accelerators, and environmental 
emergency works, such as harvesting and disposal of 
algae, and so forth. Many of these operations have heavy 
environmental impact, alter ecosystems and are often 
conducted without studies sufficient to predict their 
effects. They also involve onerous maintenance that 
communities do not always manage to carry out correctly 
and on time. Hence many of these measures very soon 
become vain and the water body lapses back into a state 
of degradation. 

 
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