Sedimentation in a shallow brackish water lagoon influenced by wind-induced waves - A methodical study

Accepted Manuscript Sedimentation in a shallow brackish water lagoon influenced by wind-induced waves A methodical study Jutta Meyer, Vivien Leonhardt, Irmgard Blindow PII:

S0272-7714(18)30320-2

DOI:

https://doi.org/10.1016/j.ecss.2019.01.005

Reference:

YECSS 6071

To appear in:

Estuarine, Coastal and Shelf Science

Received Date: 16 April 2018 Revised Date:

1 December 2018

Accepted Date: 9 January 2019

Please cite this article as: Meyer, J., Leonhardt, V., Blindow, I., Sedimentation in a shallow brackish water lagoon influenced by wind-induced waves - A methodical study, Estuarine, Coastal and Shelf Science (2019), doi: https://doi.org/10.1016/j.ecss.2019.01.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Title: Sedimentation in a shallow brackish water lagoon influenced by wind-induced

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waves - a methodical study

3 Authors: Jutta Meyer*a, Vivien Leonhardta and Irmgard Blindowa

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a

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Ernst-Moritz-Arndt University of Greifswald

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Biologenweg 15

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D-18565 Kloster / Hiddensee

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*

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[email protected]

corresponding author

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Germany

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Biological Station of Hiddensee

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ACCEPTED MANUSCRIPT Abstract In shallow aquatic ecosystems, wave-induced water motions affect the whole water column

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down to the sediment. To estimate the influence of these motions on short-term

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sedimentation rates (SR) in a shallow wind-exposed lagoon, we used plate traps (PTs)

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which, in contrast to cylindrical traps (CTs), allow trapped matter to become resuspended.

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In a series of experiments, we varied distance to the sediments, water depth, incubation

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time, and studied the SRs of total suspended matter and of suspended organic matter at

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varying conditions of wave exposure. The coefficient of variation of SRs did not change

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with incubation time. While SRs were similar on both trap types at very low wave

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exposure, they decreased with increasing wave exposure on the PTs probably due to

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instantaneous resuspension. In the CTs, SRs increased with increasing wave exposure. This

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resulted in about 55 times higher SRs in the CTs than on the PTs at high wave exposure.

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We conclude that CTs are not suitable to estimate natural SRs in wave-affected waters as

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they overestimate SRs. Our results indicate that SR obtained by PTs, which were here used

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for the first time in a wind-exposed, non-tidal ecosystem, are a far better estimate for

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natural short-term SRs.

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Keywords

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shallow coastal water lagoons; Darss-Zingst Bodden chain; sedimentation; plate traps;

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wind-induced waves; water movements

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Abbreviations cylindrical trap

CV

coefficient of variation (mean divided by standard deviation)

d

water depth [m]

d.m.

dry mass [g]

dT

water column above the trap [m]

fR

organic fraction of surface sediment [%]

fS

organic fraction of suspended matter [%]

fT

organic fraction of trapped matter [%]

λ

wave length [m]

(λ / 2) / d

wave exposure of sediment traps or of the surface sediment: half the wave

or

length divided by the water column above the sediment or above the

(λ / 2) / dT

sediment traps

PT

plate trap

SOM

concentration of suspended organic matter [mg d.m. L-1]

SR

sedimentation rate of total suspended matter [g d.m. m-2 h-1]

SR-OM

sedimentation rate of suspended organic matter [g d.m. m-2 h-1]

TSM

concentration of total suspended matter [mg d.m. L-1]

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1. Introduction During sedimentation, suspended matter settles down from the water column, while this process is reversed during resuspension (Søndergaard et al., 1992; Duin et al., 2001).

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Both processes have an important effect on turbidity and thus light availability in the water

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column and therefore on macrophytes and phytoplankton (Jeppesen et al., 1999). Both

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processes also affect nutrient cycling, as bioavailable nutrients are stored in the interstitial

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of the sediment and can be released by resuspension (Søndergaard et al., 1992).

Shallow estuaries are largely dominated by sediment resuspension due to wind-

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induced waves (Kristensen et al., 1992; Laenen & LeTourneau, 1996; Hofmann et al.,

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2011). Surface gravity waves can “touch” the sediment surface when water depth (d) is

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smaller than half the wave length (λ / 2) (CERC, 1984). Whether such waves induce a

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sediment resuspension, depends on the sediment type (Ziervogel & Bohling, 2003) and on

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the presence or absence of a sediment consolidation by extracellular polymetric substances

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(Tolhurst et al., 2002; Gerbersdorf et al., 2004).

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In contrast to running waters and deeper standing waters, where water movements are

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less complex and often unidirectional, sedimentation and resuspension are hard to measure

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in shallow wind-exposed habitats. Different methods have been applied to obtain at least

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rough estimates of resuspension rates (for a review, see Bloesch, 1994). Erosion chambers

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were used to measure resuspension rates in shallow lakes to study the influence of

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sediment consolidation on resuspension thresholds (Kleeberg et al., 2013). Changes in

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concentrations of suspended matter and light attenuation were measured to model

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resuspension events induced by wind conditions in whole lakes (e. g. Hofmann et al., 2011;

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Nicolodi et al., 2013). Sedimentation rates were determined by means of sediment traps of

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different shape (Gardner, 1980a). Reed (1989) introduced a method for sampling settled

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matter in floodplains using a filter paper (diameter: 9 cm) secured to a plastic disc which

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was laid on the marsh surface. This method was refined by Schoelynck et al. (2015), and

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Maynard et al. (2011) used a similar sediment trap in the saltmarsh of the Eden Estuary,

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Scotland, to study effects of transplanted vegetation on sedimentation and resuspension.

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Another kind of flat trap, later called plate trap (PT), was applied in both sheltered and

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running waters (Kozerski & Leuschner, 1999; Banas & Masson, 2003). Here, we estimate sedimentation rates and resuspension rates by the comparison of

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cylindrical traps (CT) and PTs. The PTs were very similar to that used by Kozerski &

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Leuschner (1999). CTs prevent resuspension of already trapped matter, if their length to

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diameter ratio (aspect ratio) is at least 5:1 (e. g. Hargrave & Burns, 1979). In contrast, plate

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traps (PTs) allow a resuspension of previously settled matter due to their flat construction

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(e. g. Kozerski & Leuschner, 2000). According to our knowledge, however, PTs have not

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yet been applied in shallow, wind-exposed, non-tidal ecosystems.

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The present study aims at testing a method to estimate short-term SRs, of both total suspended matter (TSM) and organic matter (SOM), influenced by wave exposure in

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shallow, wind-influenced aquatic ecosystems, which ultimately will contribute to

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understand and estimate the fluxes of particulate matter and nutrients in such habitats. We

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compared PTs and CTs in combined trap systems. In a series of experiments, we

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investigated the influence of incubation time, distance to the sediment and distance to the

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water surface on SRs. We analysed how the SRs, gained by the two trap types, changed

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with the amount of suspended matter. We also calculated the variability of SRs at similar

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conditions. We assumed that the SRs, gained by PTs, were affected by wave-induced

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resuspension of just settled matter, whereas the SRs, gained by CTs, were not affected by

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resuspension. We analysed trapped TSM and SOM separately as we assumed that SOM

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has a lower density than suspended inorganic matter and therefore sinks slower and

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resuspends more easily.

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2. Methods

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2.1 Site description

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The Grabow is one of four shallow, coastal, phytoplankton-dominated lagoons of the Darss-Zingst Bodden chain, located at the southern Baltic Sea (Fig. 1) with a minimum and

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maximum salinity of 3.2 and 14.3, respectively (Schumann et al., 2006). It has an area of

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41.5 km2, a mean water depth of 2.3 m, and as it is a part of the Baltic Sea, no tidal

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elevation exists (Schuman et al., 2006). All experiments were performed at a location

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approximately 170 m off the south-west shore (N54°21’58.2’’ / E12°48’25.2’’, Fig. 1), in a

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water depth between 0.90 m and 1.65 m, from July to September 2014. At the study

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location, fetch values (= the distance along which wind can blow unobstructed over the

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water surface) were as low as 170 m and 455 m at wind directions from west and south,

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respectively, but reached 6,625 m and 8,200 m at wind directions from north-northeast and

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northeast, respectively (Fig. 1).

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Fig. 1 Southern Baltic Sea (a) with the island of Rügen, the Darss-Zingst Bodden chain

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(DZBC) and the Grabow lagoon. In the detailed view of the Grabow lagoon (b), the

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location of the trap incubation is situated at the centre of the arrows (“traps”;

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N54°21’58.2’’ / E12°48’25.2’’). The arrows indicate the fetch, i. e. the distances of water

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surface over which the wind can blow unobstructed. Its values are noted next to the

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corresponding nine wind directions (Source: OpenSeaMap).

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2.2 Sedimentation traps One plate trap (PT) and one cylindrical trap (CT; diameter: 0.05 m and length:

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0.325 m) were fixed at the same “guide”, thus forming one entity (“trap system”; Fig. 2).

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The collecting area of the PT and the opening of the CT were on the same horizontal level.

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The guide could be moved up and down on a metal pole which was fixed in the sediment.

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The metal pole was 2 m long, limiting the water depth to 1.5 m for trap exposure. This

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rather low water depth is nonetheless representative for the lagoon as 24% of the lagoon’s

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area has a water depth less than 1.5 m (bathymetry data: State Agency for the

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Environment, Nature and Geology of Mecklenburg-Western Pomerania and the Federal

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Maritime and Hydrographic Agency). Each PT had a plane circular plate with a diameter

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of 0.35 m where suspended particles could settle. This circular plate was divided into two

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pieces. The outer ring (width: 0.1 m) was supposed to establish uniform hydrodynamic

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conditions when using the PT in a unidirectional flow (Kozerski & Leuschner, 2000). The

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inner disc served as a collecting area (diameter: 0.15 m, area: 0.0225 m2). The collecting

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area was designed as a piston which could be moved up and down within a cylinder. This

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movement was forced by manually pumping water into the cylinder by using a hose and a

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manual, peristaltic pump. For recovery of the settled matter, the collecting area was

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lowered into the cylinder and a cover, which moved simultaneously, closed the cylinder.

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The cover was fixed 3 cm above the collecting area with a metal pin. The settled matter

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could thus be recovered without losses and was transferred into a bottle until further

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analysis.

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Flume tests (Kozerski & Leuschner, 1999) showed that the cover did not hinder particles to settle on the collecting area beneath the cover, when horizontal water 7

ACCEPTED MANUSCRIPT movements were larger than vertical movements. In rivers, the current velocity is normally

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higher than the sinking velocity of particles. For shallow water waves, the horizontal

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component of water particle velocity does not change between water surface and bottom

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except for the bottom boundary layer, whereas the vertical component of particle velocity

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decreases from the water surface to the bottom (Sorensen, 2006). Thereby, the horizontal

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component of particle velocity is higher than the vertical component at wave motions. a)

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Fig. 2 Side view on an open (a) and closed (b) PT. (b) A trap system with one PT and one

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CT fixed at the same guide. (c) The PT is fixed at a metallic guide which can be moved up

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and down along a metallic pole.

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2.3 Experimental design

We performed four experiment series with varying incubation time, distances between

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traps and water surface, and distances between traps and sediment surface (see Table 1).

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During each of the seven “24 h-series” experiments, four trap systems were placed for 24 h

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at the same water depth and with the same distance between trap and sediment surface. We

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calculated the coefficients of variation (CV) for each experiment, which were called

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afterwards as “reference values”. During each of the six “time-series” experiments, four

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trap systems were placed in the water column at the same water depth and with the same

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distance between trap and sediment surface. The incubation duration ranged from 6 h to 8

ACCEPTED MANUSCRIPT 72 h. During each of the four “height-gradient-series” experiments, four trap systems were

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placed for 24 h in the same water depth, but the position of the traps on the pole varied

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(Fig. 3a), causing varying distances between traps and water surface as well as traps and

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sediment surface. The lowest trap system consisted of one PT alone to investigate the

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sedimentation behaviour near the sediment surface. During each of the four “depth-

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transect-series” experiments, four trap systems were placed for 24 h at varying water depth

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(Fig. 3b). Here, the distances between the traps and sediment surface were constant, but

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distances between traps and water surface varied.

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Table 1 Characteristics of the different experiment series. experiment

24 h-series

time series

height gradient

depth transect

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6

4

4

0.90-1.07

1.45-1.51

0.93-1.50

0.34-1.37

0.39-0.95

series No. of experiments

height of the water column

0.77-1.09

height of trap

0.47-0.70

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system (I-IV)

0.35-1.0

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above the traps (dT) [m]

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water depth [m]

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above the

I: 0.14 (only PT) 0.35-0.44

sediment [m] exposure duration

ca. 24 h

II: 0.76 III: 0.88

0.51-0.60

IV: 1.11 6 h, 2x 12 h, 24 h, 48 h, 72 h

ca. 24 h

ca. 24 h

aims:

coefficients

quality of

influence of wave exposure both on

determination of

of variation

sedimentation

SR and SR-OM, additionally,

(CVs)

rates

influence of sediment surface on both factors (height gradient series)

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Fig. 3 Experimental design during the two experiment series “height-gradient” (a) and

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“depth-transect” (b). PTs are indicated as an ellipse and CTs are indicated as a cylinder.

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Values for dT are indicated. b)

Depth-transect experiment

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a) Height-gradient experiment

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At the beginning and end of each experiment, the actual water depth and the height of the water column above each trap system were measured at the metal pole. For

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quantification of seston as TSM and SOM, three replicate water samples were taken

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through the whole water column with a Plexiglass tube (length: 1.5 m, inner diameter:

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6.4 cm). Pre-and post-experimental values of all parameters were averaged in order to

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represent the conditions during the experiments.

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All samples were filtered through pre-combusted filters. The settled matter was prefiltered through MGM filters (Munktell, ø 100 mm), and the filtrate was then GF/C-filtered

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(Whatman, ø 48 mm). The seston samples were GF/C-filtered. To obtain dry mass of total

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settled matter and of settled organic matter, the filters were dried for 24 h at 105°C and

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combusted for 4 h at 525°C. To calculate SR and SR-OM, we followed the methods

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described by Gasith (1975) and Callieri et al. (1991) and subtracted the concentration of

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suspended matter in the traps off the dry mass of trapped matter (equation 1), as the CT

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volume (687 ml) was higher than the PT volume (530 ml).

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.

.

ℎ

!!"#

=

!

"

$ %$"%

" ∗&%$,-

& %

'()∗

!+ , "

& % & "∗ ...

[1]

2 3

With dry mass of trapped matter [mg d.m.], concentration of suspended matter in the water column [mg L-1], trap volume [L], the area [m2] of the collection disc of the PT as

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well as of the CT bottom and the incubation time [h]. The same equation was used to

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calculate SR-OM, by substituting “mass of total trapped matter” by “mass of trapped

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organic matter” and substituting “TSM” by “SOM”. We also calculated quotients of the

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sedimentation rates and the concentrations of suspended matter (SR / TSM and SR-OM /

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SOM). Additionally, we calculated resuspension rates (equation 2) according to Gasith

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=

∗

( 0

1)

( 3

1)

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(1975; see also Bloesch, 1994):

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[2]

With R: resuspension rate of surface sediment [g d.m. m-2 h-1], fS: organic fraction of

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trapped matter [%], fT: organic fraction of suspended matter (=SOM [%]) and fR: organic

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fraction of surface sediment (0.8%; Forster & Bitschofsky, 2015). In the following text, we

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will call the term (fS – fT) / (fR – fT) “organic fraction factor”.

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Wind speed, wind direction and water level were recorded hourly at Barth (distance to

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study site: 5 km) during the whole experimental period. Weather data were provided by

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Deutscher Wetterdienst (www.dwd.de) and water depth gauge readings were provided by

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Wasserstraßen- und Schifffahrtsverwaltung des Bundes (www.pegelonline.wsv.de). The

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distance from the installed traps to the opposite shore (fetch) was measured for all wind

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directions at every 22.5°, and the mean wave length (λ) was calculated for the opening

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duration of every trap system according to Laenen & LeTourneau (1996; see also CERC,

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1984). The water depth varied during the experiments due to wind-influenced water level 11

ACCEPTED MANUSCRIPT changes. These fluctuations (-0.3 to +0.2 m relative to normal water level at the measuring

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station at Barth) were taken into account when water depth (d) and water column above the

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traps (dT) were calculated. To describe the potential influence of waves (= wave exposure),

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hourly calculated half the wave lengths (λ / 2) were divided by dT and averaged for each

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trap system over the incubation duration.

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2.5 Statistical analyses

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For statistical analyses, we used the program SPSS Statistics 22. Comparisons were

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performed with Wilcoxon and Mann-Whitney-U tests as the number of individuals were

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mainly rather small (three to five individuals). Correlations were performed with

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Spearman’s rho two-tailed test as most of the data were not normally distributed.

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Results

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3.1 Wind conditions, surface wave length, total suspended and suspended organic matter

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Wind speed reached mainly from calm to a gentle breeze (0 - 5.4 m s-1, Beaufort scale 3) and was higher than 5.4 m s-1 (up to 11.6 m s-1) at 17% of all hourly wind speed

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measurements (Fig. 4). The dominant wind direction was east with a corresponding fetch

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of 4,650 m (Fig. 4). Wind from N to SSE, which was accompanied by high fetch values

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(1,885 - 8,200 m; see also Fig. 1), was recorded during 54% of the whole incubation time

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(Fig. 4). During the total experimental time period, calculated wave lengths varied between

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0.15 m and 3.48 m. Theoretically, the wave movements reached down to the sediment

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surface at 15% of all measurement days ((mean wave length / 2) / mean water depth).

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Fig. 4 Conditions of wind speed and direction during the measurements. Source of data:

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DWD.

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Average (± SD) values for TSM and SOM were 55 ± 6 mg L-1 and 19 ± 6 mg L-1,

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respectively, resulting in a mean (± SD) organic fraction of 36 ± 8% during the whole

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experimental period (Fig. 5). Overall, TSM did not correlate with wave lengths

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(p = 0.892), whereas SOM and the organic fraction were negatively correlated with wave

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length (SOM: ρ = -0.514; p = 0.024 and organic fraction: ρ = -0.527; p = 0.014) with wave

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length (Spearman’s rho of n = 19 each).

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2

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TSM & SOM [mg L-1]

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Wave length [m]

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Fig. 5 TSM ( ) and SOM ( ) during the experimental period at the study location plotted

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against wave length.

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3.2 Twenty-four-hour-series and time-series

The mean wave exposures were low during the experiments of the “24 h-series”

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((λ / 2) / dT = 0.03 - 0.70), except during the second experiment ((λ / 2) / dT = 0.92 - 1.12;

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Fig. 6). The mean CV (Fig. 6) was 29% each of SRPT, SRCT, SR-OMPT and SR-OMCT and

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was used as reference CV.

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In the “time-series” experiments, the CVs of the SRs and SR-OM of both trap types

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did not correlate with incubation time (Spearman’s rho: p = 0.103 - 1.000; n = 5 - 6; Fig.

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6c, d). The wave exposure was high ((λ / 2) / dT > 1) during the first 12 h-experiment and

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at the beginning of the 48 h experiment, but low ((λ / 2) / dT < 1) during the other

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experiments. The CVs of neither SR nor SR-OM differed from the corresponding reference

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CVs (Wilcoxon: p = 0.086 - 1.000). 24 h-series 14

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13 30 13

2

84 56 8 28 40 29 11 3 3

0 1

2

3

4

5

6

1.5 1 0.5

17 38 31

0 1

7

Number of experiments

21 5

3 9 18 46 28

27

48 h

24 h

12 h_2

12 h_1

6h

0

Name of experiment

50 40 30 20 10 0

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9 17 6h

50

4

5

6

7

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SR-OM [g d.m. m-2]

150 100

3

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SR [g d.m. m-2]

Time series d) 60

6

200

2

Number of experiments

72 h

c)

84 41 58 24 11 23 8 5 26 25

41

5 30 1315 6 72 h

4

77

2

48 h

6

2.5

24 h

8

12

12 h_2

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12 h_1

SR [g d.m. m-2 h-1]

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SR-OM [g d.m. m-2 h-1]

a)

Name of experiment

Fig. 6 Mean (± SE, n = 2-4) SR (a and c) and SR-OM (b and d) on PT ( ) and in CT ( )

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in the seven “24 h-series” experiments (a and b) and in the six “time-series” experiments (c

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and d). The CVs are noted as numbers above the corresponding columns.

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3.3 Height-gradient- and depth-transect-series

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The wave exposure of the sediment was low during the four experiments of the height-

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gradient-series (Fig. 7a, b). The SRs on PTs closest to the sediment surface (trap system I)

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were 2-3 (experiment 2-4) and 19 (experiment 1) times higher than that of the PTs of the

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trap system II which was located 0.76 m above the sediment surface, in the middle of the

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whole water column. Combining the four experiments, the trapped matters (SR and SR-

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OM) were neither in PTs nor is CTs correlated with wave exposure (Spearman’s rho:

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p = 0.208 - 0.499; n = 12 - 16). As the wave exposure was overall low, we correlated the

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trapped matter with dT. A positive correlation with dT was found for SR and SR-OM on the

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PTs (Spearman’s rho for SR: ρ = 0.714, p = 0.002; n = 16 and for SR-OM: ρ = 0.730; 15

ACCEPTED MANUSCRIPT p = 0.001; n = 16), whereas there were no correlations for CTs (Spearman’s rho: p ≥ 0.562;

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n = 12). The CVs of SR and SR-OM gained on the PTs were higher than the reference CVs

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(Wilcoxon: p = 0.024 and p = 0.012, respectively), whereas the CVs of SR and SR-OM

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gained in the CTs did not differ from the corresponding reference CVs (Wilcoxon:

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p ≥ 0.412).

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In experiments 1-3 of the “depth-transect” series (Fig. 7c and d), the wave exposure was high, but it was overall low during experiment 4. When combining the four

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experiments, the SR on the PTs did not correlate with wave exposure (Spearman’s rho:

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p = 0.123; n = 16), whereas SR-OM on the PTs correlated negatively with wave exposure

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(Spearman’s rho: ρ = -0.679; p = 0.007; n = 15). In contrast, the SR and SR-OM in the CTs

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correlated positively with wave exposure (Spearman’s rho: ρ = 0.538; p = 0.034; n = 16

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and ρ = 0.597; p = 0.017; n = 16, respectively). The CVs of SR and SR-OM gained by PTs

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were higher than the reference CVs, whereas there were no differences regarding CTs

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(Wilcoxon: p = 0.0424, p = 0.016 and p ≥ 0.527, respectively; Table 2).

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0.1

0.01 0.2

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Depth-transect-series d) 10

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0.1

0.2

0.4

0.6

2

3

0.8

1

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10 SR [g d.m. m-2 h-1]

0.01

0.001 0

c)

0.1

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SR [g d.m. m-2 h-1]

10

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a)

Height-gradient-series b) 1

0.1

0.01

0.001

0.01 0

1

2

3

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wave exposure (λ / 2) / dT

4

0

1

4

wave exposure (λ / 2) / dT

Fig. 7 SR (a and c) and SR-OM (b and d) on PT (black symbols) and in CT (grey symbols)

2

in the four “height-gradient” experiments (a and b) and the four “depth-transect”

3

experiments (c and d) plotted against wave exposure. Values for the same experiment are

4

connected by lines. Symbols: first experiment ( ), second experiment ( ), third

5

experiment ( ), fourth experiment ( ). Unfilled symbols are used to illustrate the lowest

6

trap (TS I).

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Table 2 CVs calculated for SR and SR-OM for both trap types (CT, PT) during the height-

2

gradient series and the depth-transect series. height gradient

depth transect

CV SR

SR-OM

SR

SR-OM

[%] 2

3

4

1

2

3

4

1

2

3

PT

162

73

104

64

168

55

116

87

74

46

52

CT

10

25

12

19

16

24

7

20

15

24

28

3

1

2

3

4

97

10

55

88

63

59

17

21

19

19

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4

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1

3.4 Combined data analysis

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When combining all experiments and trap types, 23% of all incubated trap systems

7

experienced high wave exposure (Fig. 8). SR and SR-OM were higher in CTs than on PTs

8

at both wave exposure categories (Wilcoxon test: each p < 0.001). At low wave exposure,

9

mean (± SD) SRs were 6 ± 8.4 times and SR-OM were 11 ± 20.4 times lower on PTs than

10

on CTs (Fig. 8a and b). In contrast, at high wave exposure, they were 55 ± 50.7 (SR) and

11

107 ± 108.7 (SR-OM) times higher in CTs than on PTs.

12

Combining both wave exposure categories, SR and SR-OM on the PTs did not correlate

13

with TSM and SOM, respectively, but both parameters correlated positively with dT and

14

negatively with wave exposure (Table 3 and Fig. 8a and b). The quotients SR / TSM, SR-

15

OM / SOM and SR-OM / SR of PTs correlated negatively with wave exposure (Table 3,

16

Fig. 8d and e). In the CTs, SR and SR-OM correlated positively with TSM, and SR-OM,

17

respectively. Both parameter and SR / TSM as well as SR-OM / SOM of CTs correlated

18

positively with wave exposure (Fig. 8a, b, d, e).

19

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In the following paragraphs, we used three ways to calculate resuspension rates. By

20

subtracting the SR of PTs off the SR of CT (as well as for SR-OM) at high wave exposure,

21

we gained theoretical resuspension rates of 2-10 g d.m. m-2 h-1 (total settled matter) and

22

0.7-3.1 g d.m. m-2 h-1 (settled organic matter), respectively. 18

ACCEPTED MANUSCRIPT 1

We assumed that no resuspension exists at the intersection points of the trend lines of PTs and CTs for SR and SR-OM, respectively, against wave exposure. We estimated

3

resuspension of SR and SR-OM, respectively, at a certain high wave exposure by

4

subtracting the trend line values at this wave exposure from the values at the intersection

5

points (see Fig. 8a, b). The resulting resuspension rates of total settled matter and settled

6

organic matter were 0.36 – 0.68 g d.m. m-2 h-1 and 0.08 – 0.18 g d.m. m-2 h-1, respectively.

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The resuspension rates of sediment, calculated with both trap types (Fig. 8 e),

8

according to equation 2 (Gasith, 1975), were negative at 4% (PT) and 24% (CT) of the

9

measurements, as the organic content of trapped matter (fS) was higher than that of

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suspended matter (fT; resulting in fs – fT > 0). Values which based on this positive term (fS

11

– fT) were neglected in the following considerations. Sediment resuspension rates,

12

calculated for PTs, decreased with increasing wave exposure (Spearman’s rho: ρ = -0.461;

13

r = < 0.001; n = 71) and were higher at low wave exposure (mean ± SD: 0.29 ± 0.38

14

g d.m. m-2 h-1) than at high wave exposure (mean ± SD: 0.04 ± 0.03 g d.m. m-2 h-1; Mann-

15

Whitney-U test: p = < 0.001). In contrast, the values of CTs did not correlate with wave

16

exposure (Spearman’s rho: ρ = -0.039; r = 0.773; n = 58) and did not differ between the

17

two wave exposure categories (overall mean ± SD: 0.50 ± 0.52 g d.m. m-2 h-1; Mann-

18

Whitney-U test: p = 0.195). The term fs – fT for both trap types and the term fR – fT

19

increased with increasing wave exposure (Spearman’s rho for PTs: ρ = 0.352; r = 0.003;

20

n = 71, for CTs: ρ = 0.425; r = 0.001; n = 58 and for fR – fT: ρ = 0.459; p = < 0.001;

21

n = 79). The organic fraction factor ((fS – fT) / (fR – fT)) did not correlate with wave

22

exposure for PTs (overall mean ± SD: 0.36 ± 0.16) while it decreased with increasing wave

23

exposure for CTs, from (mean ± SD) 0.23 ± 0.12 to 0.10 ± 0.07 at low and high wave

24

exposure, respectively, (Spearman’s rho for PTs: r = 0.064; n = 71 and for CTs: ρ = -0.457;

25

r = < 0.001; n = 58), probably because the term fR – fT increased more than the term fS - fT.

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ACCEPTED MANUSCRIPT

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1 Fig. 8 SR (a), SR-OM (b), SR / TSM (c), SR-OM / SOM (d) and resuspension rate (e) on PT ( ) and in CT ( ) of all experiments were plotted against wave exposure. The resuspension rate was calculated according to equation 2 (Gasith, 1975). Dotted lines show trends for PTs (black) and CTs (grey). 20

ACCEPTED MANUSCRIPT 1 2

Table 3 Results of Spearman’s rho correlation tests for relation comparisons between TSM mg L-1 and SR g d.m. m-2 h-1 and between SOM mg L-1 and SR-OM g d.m. m-2 has well as between dT [m] and wave exposure ((λ / 2) / dT) and the following parameters:

1

4

SR, SR-OM, SR / TSM and SR-OM / SOM. The tests were done for all wave exposures

5

combined. Correlations with SR and SR-OM were significant at p < 0.017 (bold values)

6

after Bonferroni correction (0.05 / 3). This correction was chosen as these depending

7

variables were compared with three independent variables (TSM or SOM, dT and wave

8

exposure). Correlations of wave exposure with the quotients SR / TSM, SR-OM / SOM

9

and SR-OM / SR were significant at p ≤ 0.05 (bold values).

TSM

SOM

p-value

n

-0.060

0.629

67

0.661

<0.001

67

0.260

0.035

66

-0.254

0.041

65

0.447

<0.001

70

-0.074

0.540

71

SR-OMPT

0.424

<0.001

66

SR-OMCT

-0.004

0.973

65

SRPT

-0.498

<0.001

70

SRCT

0.460

<0.001

71

SR-OMPT

-0.510

<0.001

69

SR-OMCT

0.452

<0.001

69

SRPT SRCT SR-OMPT SR-OMCT

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SRPT dT [m]

ρ

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Wave

SR-OMPT / SRPT

-0.268

0.025

70

exposure

SR-OMCT / SRCT

-0.140

0.245

71

SRPT / TSM

-0.557

<0.001

71

SRCT / TSM

0.328

0.005

71

SR-OMPT / SOM

-0.397

0.001

68

SROMCT / SOM

0.405

0.001

67

10

21

ACCEPTED MANUSCRIPT 1 2

4. Discussion Sedimentation is hard to quantify in large, shallow, dynamic waters (e. g. Kozerski, 1994; Kozerski, 2006), as wave-induced water movements affect the whole water column

4

and therefore, sedimentation alternates with resuspension during very short time periods.

5

In contrast to the commonly used CTs, PTs allow a resuspension of newly settled matter.

6

In our study, it was the first time that PTs were used to estimate SR in an aquatic

7

environment, where wave-induced water movements often influence the whole water

8

column.

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As the aims of this study were methodological oriented, the trap incubations took

10

place only during summer. Therefore, the resulting short term sedimentation rates of total

11

suspended and suspended organic matter were not representative for a whole year.

12

Sedimentation rates during the other seasons may be higher or lower due to different wind

13

/ wave conditions. The poles at which the traps were installed were only 2 m long. This

14

limited the total water depth for trap incubation. Longer poles (e. g. 3-4 m long) may have

15

needed a kind of concrete basement, which has to be embedded in the sediment, due to

16

increasing stronger leverage effects during trap handling. However, this effort was not

17

realizable in this project. We wanted to analyse effects of different positions within the

18

rather low water column, and positioned the trap, when possible, randomly along the poles.

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4.1 Water movement characteristics In the study area, water movements are induced by in- and outflow events as well as

22

by wind, resulting in currents, surface waves. In our study, we had only the possibility to

23

calculate wave length of the surface waves by using data of wind speed and direction to

24

determine wave exposure of the traps, We occasionally measured particle velocities with a

25

Doppler velocity meter (Vector, Nortek) during the experiments, revealing no significant

26

current velocities but orbital velocities indicating waves (unpublished results). However, 22

ACCEPTED MANUSCRIPT 1

we could not determine the effects of in- and outflow on water currents at the studied

2

location in the lagoon.

3

Specific wind and surface wave conditions induce a special form of complex water motions, the so called Langmuir circulations (Langmuir, 1938). Langmuir circulations

5

occur regularly in the studied lagoon (Chubarenko et al. 2010), and as they affect the

6

whole water column there (Schubert et al., 2001), they can theoretically induce

7

resuspension (Gargett et al., 2004). We had no possibility to quantify effects of Langmuir

8

circulations on the water motions above the traps, but the fact that the amounts of settled

9

matter caught on the PTs decreased with increasing wave exposure already at surface

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4

waves that could theoretically not reach the trap’s surface indicates that Langmuir

11

circulation may be a cause for resuspension of settled matter on PTs. No laboratory studies

12

with PTs have yet been performed under wave conditions and are badly needed to support

13

and validate the use of these traps in natural wave-influenced habitats.

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4.2 Influence of incubation time, suspended matter, wave exposure and incubation depth as

16

well as trap construction on sedimentation rates In the time-series experiment, coefficients of variation of sedimentation rates did not

18

change with incubation time. We therefore assume that the sedimentation rate estimation

19

do not become more reliable with increasing incubation time. In line with our observations

20

that wave conditions varied considerably during the 48 h time series experiment, we advise

21

to apply short incubation times in experiments estimating instantaneous wave effects. Also

22

experiments with long incubation times might be useful to study the consolidation of

23

settled matter and its effects on the susceptibility to resuspension.

24

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Both in the CTs and the PTs, the share of organic matter in total settled matter was far

25

higher than in the sediments around the sediment traps (0.8% of dry mass; Forster &

26

Bitschofsky, 2015). Müller (2002) assumed that in the adjacent lagoon Barther Bodden, the 23

ACCEPTED MANUSCRIPT low concentration of organic matter in the sediment is a result of a fast recycling, a

2

substantial resuspension of organic matter or a near sediment transport to sheltered and

3

deeper locations. An additional explanation may be fast transport of this matter into the

4

adjacent the open Baltic Sea during large-scale water level fluctuations as described e. g.

5

by Ekman (2009).

6

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As the organic content of the sediment at the studied location is very low,

consolidation might not happen at wave-exposed locations. More sheltered locations might

8

be more suitable for those experiments. However, the use of PTs at sheltered locations may

9

be biased by the plate’s cover. When particles sink more vertically than diagonally, they

10

will settle on top of the cover instead of beneath it. Banas & Masson (2003), found 1-3

11

times higher sedimentation rates on uncovered than on covered plate traps which were

12

positioned within stands of submerged macrophytes. The macrophytes may impede

13

horizontal water movements and thus retain trapping of vertically settling matter by the

14

cover (Banas & Masson, 2003). Hargrave & Burns (1979) obtained small differences

15

between sedimentation rates of covered and uncovered cylindrical traps at horizontal water

16

velocities some orders of magnitudes higher than the sinking velocities of particles. We

17

hope that future laboratory studies, e. g. with Particle Image Velocimetry, will reveal at

18

which wave movements the cover reduces particle settling on the traps.

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In previous laboratory studies, PTs have been tested in several current flume

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20

experiments with rather contradictory results. While Gardner (1980a) showed that a flat

21

Plexiglas plate had an efficiency to collect settling matter of only 31% at a flow velocity of

22

9 cm s-1, Kozerski & Leuschner (2000) showed that PTs, almost identical to ours, obtained

23

reliable sedimentation rates at current velocities up to 15 cm s-1. These findings underline

24

the need for further current and wave flume studies.

25 26

We had expected that the sedimentation rates were positively correlated with the corresponding fractions of suspended matter in the water column in both trap types, but 24

ACCEPTED MANUSCRIPT found such a correlation only for the total matter in the CTs. It seems that the influence of

2

wave exposure on sedimentation rates is more pronounced than effects of amounts of

3

suspended matter. The decreasing wave exposure at increasing water depth resulted in

4

huge differences of sedimentation rates on the PTs incubated at different positions in the

5

water column. We conclude that PTs should be installed as close to the sediment surface as

6

possible to obtain estimates for natural sedimentation rates. At locations with an existing

7

fluffy layer, however, plate traps should be kept above this layer. As its particles already

8

have settled but are moved by water motions (e. g. Leipe et al., 2000), they can be moved

9

onto PTs erroneously.

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As expected, sedimentation rates of both total and organic matter on the PTs decreased

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1

with increasing wave exposure, indicating a resuspension of already trapped matter. In

12

contrast, the quotient of sedimentation rate and suspended matter increased with increasing

13

wave exposure in the CTs, indicating an over-collection of suspended matter at high wave

14

exposure. This over-collection might be caused by an active transport of suspended matter

15

into the CT due to turbulences around its opening (Gardner, 1980a), and it indicates that

16

CTs are unsuitable to measure reliable sedimentation rates in wave-exposed habitats. Due to this over-collection of CTs, it is not possible to calculate resuspension rates by

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subtracting sedimentation rates of PTs from that of CTs at high wave exposure, as the

19

resulting resuspension rates would be over-estimated as well.

20

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The application of the equation of Gasith (1975) resulted in negative resuspension

21

rates (CTs), and resuspension rates calculated for PTs decreased with increasing wave

22

exposure, which of course is a bias and does not reflect the natural situation. The negative

23

resuspension rates are explained by the fact that the organic content of the trapped matter

24

in the CTs often was higher than in the surrounding water, suggesting a selective trapping

25

of organic matter. In opposite to several Finnish lakes, where the method of Gasith (1975)

26

results in resuspension rates that almost equal sedimentation rates of TSM (Horppila & 25

ACCEPTED MANUSCRIPT 1

Nurminen 2001) or are somewhat lower (Horppila & Nurminen 2003), this method seems

2

not applicable in the wind-exposed lagoon investigated by us.

3 4

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ACCEPTED MANUSCRIPT 1

5. Conclusions Determination of sedimentation rates and especially of organic matter in shallow

3

wave-influenced waters is important, as the exchange processes between water column and

4

sediment highly affect whole-ecosystem nutrient budgets and turbidity. Our pilot study

5

indicates that plate traps are a promising tool to estimate sedimentation rates in this habitat,

6

as they reflect the instantaneous influences of wave motions on sedimentation of

7

suspended matter more realistically than the, up to now, more commonly used cylindrical

8

traps. We assume that sedimentation rates obtained by plate traps can approach natural

9

sedimentation rates (= sedimentation - instantaneous resuspension) at higher wave

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2

exposure. In contrast, we assume that cylindrical traps overestimate the vertical flux of

11

both total suspended and organic suspended matter due to an active transport of particles

12

into the traps. We highly recommend to test plate traps under laboratory conditions in a

13

wave flume to determine possible cover effects on the settling behaviour of particles at

14

both high and low wave exposure. We recommend to install the plate traps close to the

15

sediment surface to gain natural sedimentation rates.

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27

ACCEPTED MANUSCRIPT Acknowledgements

2

We thank Hendrik Schubert (University of Rostock) for supporting this investigation,

3

Hans-Peter Kozerski, Jan Köhler (Leibniz-Institute of Freshwater Ecology and Inland

4

Fisheries), Stefan Thoma and Sabine Wilczek (SGL Spezial- und Bergbau-

5

Servicegesellschaft Lauchhammer mbH) for lending the sedimentation traps including

6

equipment as well as for their technical support. We thank Maria Schiffler and Laura

7

Schulz for their assistance during field and laboratory work as well as the teams of the

8

Biological Station of Zingst (University of Rostock) and the Biological Station of

9

Hiddensee (University of Greifswald) for laboratory and technical support. We appreciate

10

the constructive criticism by four anonymous referees which considerably improved this

11

paper.

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12 Funding

14

The study was part of the project “Baltic Coastal System Analysis and Status Evaluation”

15

and therefore, financially supported by the German Federal Ministry of Education and

16

Research (project number: 03F0665 C).

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wave equations and wave characteristics, pp. 9-52, Fig. 2.2. Springer Science and

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Business Media Inc, New York. http://www.springer.com/978-0-387-23332-1 32

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Tolhurst, T. J., G. Gust, D.M. Paterson, 2002. The influence of an extracellular polymeric

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substance (EPS) on cohesive sediment stability. Proceedings in Marine Science, 5:

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409–425. Ziervogel, K., B. Bohling, 2003. Sedimentological parameters and erosion behaviour of

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submarine coastal sediments in the south-western Baltic Sea. Geo-Marine Letters,

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23: 43–52, doi: 10.1007/s00367-003-0123-4.

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ACCEPTED MANUSCRIPT Highlights 1. Plate (PTs) and cylindrical traps (CTs) were applied in a shallow wave-exposed lagoon. 2. PTs allow water movements to resuspend instantaneously settled matter.

RI PT

3. Amounts of trapped matter on the PTs decreased with increasing wave exposure.

4. PTs give reliable sedimentation rates and allow for an estimation of resuspension.

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5. CTs overestimated sedimentation at high wave exposure.