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ESTUARINE, COASTAL AND SHELF SCIENCE

Volumes 102–103, 1 May 2012, Pages 1-10




LONG-TERM CHANGES IN SECCHI DEPTH AND THE ROLE OF PHYTOPLANKTON IN EXPLAINING
LIGHT ATTENUATION IN THE BALTIC SEA

Author links open overlay panelVivi Fleming-Lehtinen a, Maria Laamanen b 1
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ABSTRACT

Secchi depth, as a proxy of water transparency, provides valuable information on
the availability of light to the underwater ecosystems. Changes in water
transparency have also been widely linked to eutrophication and phytoplankton
biomass. This study aimed to describe the development of water transparency in
the Baltic Sea through a unique century-long set of Secchi depth observations.
Furthermore, the aim was to explain the role of phytoplankton in determining
water transparency in these optically complex waters. Water transparency in the
open Baltic Sea has decreased during the last one hundred years. The development
differs between the sub-basins of the Baltic Sea. The decrease has been most
profound in the north-eastern Baltic sub-basins, but apparent also in parts of
the Southern and Central Baltic. In many of the northern areas the decrease has
accelerated during the last decades, whereas in the Southern Baltic a recent
increase was observed. The analysis of simultaneous chlorophyll a observations
during the period from 1972 to 2006 revealed that during summer time, 13–17% of
the light attenuation can be attributed to phytoplankton. In spring, the average
proportion is between 31 and 42%, with great variation between observations. We
find Secchi depth a suitable indicator of eutrophication, integrating various
organic matter related features. It should always be applied with sufficient
background information of the optical properties of the water mass, and
complemented by other indicators.


INTRODUCTION

Marine eutrophication, defined as an increase of the rate of the supply of
organic matter to the system (Nixon, 1995), has caused widespread concern
leading to an increasing interest in measuring and monitoring water quality.
Although water transparency is a result of multiple parameters affecting the
attenuation of light, it has been widely linked to phytoplankton increase, and
used as an indicator of eutrophication in both marine and fresh water
environments around the world (Carlson, 1977; Lewis et al., 1988; Karydis, 2009;
Chen et al., 2010). Secchi depth also provides a simple measure of the range of
the photic water layer, of the extent of the bottom area with a light supply,
and the amount of habitat of primary producers.

The white Secchi-disc is among the few early hydrological measuring devices
still in use. This method of measuring the water transparency of natural waters,
by estimating the depth where a white disc no longer is visible to the eye, was
developed by Pietro Angelo Secchi in 1865 (Secchi, 1866). It is inversely
affected by the attenuation of light penetrating into water as governed by its
absorption and scattering properties (Preisendorfer, 1986). In sea-water,
suspended particulate matter, coloured dissolved organic matter and living
planktonic organisms, mainly phytoplankton, add to the attenuation of light
(Lund-Hansen, 2004).

In order to investigate the relative effect of suspended algal biomass on water
transparency, Megard and Berman (1989) simultaneously measured Secchi depth and
chlorophyll a in the Mediterranean Sea, finding a linear correlation in offshore
samples. The open Mediterranean Sea is an example of so-called Case I waters,
characterized as clear, optically simple, usually oceanic offshore waters, in
which the matter causing attenuation of light is governed by autotrophic
phytoplankton. The same is generally not true for optically more complex waters
described as Case II, where dissolved and particulate organic and inorganic
substances have a major effect on water clarity. In such Case II waters, the
applicability of Secchi depth as one of the eutrophication indicators can be
scrutinised through estimating the contribution of organic matter, and
especially of algal biomass to water transparency.

Secchi depth observations have been collected in the Baltic Sea since 1903,
resulting in an extensive long-term dataset covering the entire sea-area (Aarup,
2002). The data provides information on the change of the state of the light
environment of the sea during a century with an immense increase in
anthropogenic pressures on the marine environment. It can thus be characterized
as unique in a global perspective. In the Baltic Sea, Secchi depth has been
developed as an indicator of eutrophication and water quality (HELCOM, 2009).

The Baltic Sea is a semi-enclosed brackish water basin with a mean depth of only
54 m. It is separated from the ocean by the narrow and shallow Danish Sounds,
leading to the North Sea via Kattegat and Skagerrak. Being at latitudes above
N54°, the sea becomes at least partly ice-covered during winter. The basin has a
north-eastward salinity- and temperature gradient and it is vertically
stratified by a permanent halocline and seasonal thermocline (Leppäranta and
Myrberg, 2009). There is substantial riverine input from the drainage area of
1,739,000 km2, shared by altogether 14 industrialized countries. The sea suffers
from a long history of anthropogenic nutrient load which greatly intensified
from the 1950s, after the full-scale industrialisation and agricultural
development of the surrounding countries. Since about the 1960s, clear signs of
eutrophication of the marine environment emerged in the Baltic, including
increased nutrient concentrations and summer phytoplankton biomass (Wasmund and
Uhlig, 2003; Fleming-Lehtinen et al., 2008) as well as intensified spring blooms
(Raateoja et al., 2005) and annual extensive late summer cyanobacterial blooms
(Kahru et al., 1994; Finni et al., 2001).

According to Sandén and Håkansson (1996), Secchi depth decreased in the Baltic
Proper from the Second World War until the late 1990s by an average rate of
−0.05 m y−1. Launiainen et al. (1989) found that the mean Secchi depth was
2.5–3 m lower in the Baltic Proper in 1969–1986 compared to the pre-war period
1914–1939, which would also mean a change of approximately −0.05 m y−1. These
earlier studies do not take full advantage of the extensive data available from
the Baltic Sea, neither did they make a distinction between the summer and
spring growth periods.

The aim of this study is to provide a deeper insight into the relation of water
transparency and chlorophyll a concentration in optically complex waters such as
the Baltic Sea, in order to give light to the applicability of Secchi depth as
an indicator of eutrophication. In addition the aim is to provide information on
the development and trends of water transparency as a depiction of the extent of
the photic underwater habitat of the primary producers during the summer period
over the last hundred years. The sub-areas of the Baltic Sea, distinguished by a
multiple spatial gradient of physical, chemical and biological qualities, are
addressed separately.


SECTION SNIPPETS


MATERIALS AND METHODS

The study area covers almost the entire Baltic Sea, comprising nine sub-basins
north-eastward from the Arkona Sea (Fig. 1). The data (11,816 observations in
total) originate from observations carried out on monitoring or research cruises
organised since the beginning of the 20th century, locations being presented in
Fig. 1. Data were compiled from a number of sources. The ICES database contained
observations from the entire Baltic Sea between the years 1903 and 2009, which
were complemented


LONG-TERM CHANGES

A significant decrease in summer-time water transparency was observed in 7 of
the 9 sub-basins over the last one hundred years (Fig. 3 and Table 1). According
to the data, the rate of decrease was most pronounced in the Northern Baltic
Proper, the Bothnian Bay, the Gulf of Finland and the Bothnian Sea. During the
period prior to 1940, considerable decrease was observed only in the Bothnian
Bay sub-area.

Recent accelerated decrease, following a period of levelling or even slight
increase in the


SECCHI DEPTH WATER TRANSPARENCY AS AN INDICATOR OF EUTROPHICATION

The presented results indicate that in a typical Case II water body, such as the
open Baltic Sea during summer, at most 40% of the matter affecting Secchi depth
consists of autotrophic phytoplankton, with the proportion generally below 17%.
Our results are in line with findings from the optically less complex North
Sea–Arkona Sea transitional zone, where autotrophic phytoplankton was estimated
to account for an average of about 32% of the total light attenuation
(Lund-Hansen, 2004). Our results


CONCLUSIONS

Of the attenuation of light affecting water transparency in the Baltic Sea
sub-regions during summer time, 13–17% is caused by phytoplankton. Secchi depth
can thus not be linked solely to phytoplankton biomass, due to it's
responsiveness to the high background attenuation characteristic to the Baltic
Sea. It can however be used to provide an integrative indicator describing a
combination of eutrophication related characteristics, together with indicators
of primary production and algal biomass.


ACKNOWLEDGEMENTS

This study relies on data collected by the former Finnish Institute of Marine
Research (presently part of the Finnish Environment Centre, SYKE) in Finland,
SMHI in Sweden, IMWM in Poland, LIAE in Latvia and CMR in Lithuania and all
countries providing data to the ICES database. We are especially grateful to
Philip Axe, Elzbieta Lysiak-Pastuszak, Juris Aigars and Aiste Kubiliute for
their kind help in providing access to these national datasets. The scientific
work is based on the contribution

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   medium light treatment and decreased at high and low treatments for both
   populations, suggesting cisco may be adapted to withstand some light exposure
   from inter-annual variability in ice coverage. Light intensity had no overall
   effect on length of incubation. Increasing light intensity decreased
   length-at-hatch in Lake Superior but had no effect in Lake Ontario. Yolk-sac
   volume was positively correlated with increasing light in Lake Superior and
   negatively correlated in Lake Ontario. Contrasting responses in embryo
   development between lakes suggests differences in populations’ response to
   light is flexible. Our results provide a step towards better understanding
   the high variability observed in coregonine recruitment and may help predict
   what the future of this species may look like under current climate trends.

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