Rapp. Comm. int. Mer Médit., 36,2001
101
CHARACTERIZATION OF PHOSPHORUS SPECIES DISCHARGED TOTHE
S.E MEDITERRANEAN BASIN FROM LANDBASED SOURCES
Mohamed A.R. Abdel-Moati
Oceanography Department, Faculty of Science, Alexandria University, Egypt - marabdelmoati@ hotmail.com
Abstract
Particulate phosphorus species were differentiated in water discharged to the SE Mediterranean basin. Despite the discharge of 1770x10
3
MT y
-1
of suspended matter to the SE Mediterranean, the area is still phosphorus limited. Variations in the quality of discharged water do
not permit the appearance of a predominant phosphorus fraction. Iron bound phosphorus (P
fe
) was enriched in the Nile discharge while
elevated P
org
levels were discharged from highly productive delta lakes and wastewater outfalls. Bioavailable phosphorus (P
ex
and P
org
)
represents 16-67% of the particulate phosphorus pool.
Key words: Phosphorus, chemical speciation, coastal waters, Levantine basin
Introduction
Phosphorus is a key element in the biogeochemical cycles of the
marine environment. Its availability in seawater is important in con-
trolling its productiv
i
t
y. The SE Mediterrenean basin is the largest sea-
water body where primary production is phosphorus-limited. T
h
i
s
unusual limitation may be due to inorganic processes adsorbing phos-
phorus onto labile iron hydroxides carried by Saharan dust and Rive
r
Nile particulate matter. In estuarine or estuarine-like ecosystems, char-
acterized by the transition from fresh to salt water environments, an
understanding of the factors that regulate the biomass and production
requires insight into the behaviour of the different forms of the targ
e
t-
ed element rather than the total concentration. Since sequential ex
t
r
a
c-
tion techniques are most promising for separating and quantifying va
r-
ious reservoirs, the aim of the present study is to evaluate the concen-
tration levels of the main phosphorus forms in suspended matter and
reconstruct a phosphorus input rate to the SE Mediterranean basin from
landbased sources. It should help in understanding phosphorus biogeo-
chemical cycling and its effect on the present day productiv
i
t
y.
Material and Methods
Water samples were collected from the major landbased sources dis-
c
h
a
rging to the SE Mediterranean basin off the Egyptian coast. T
h
e
s
e
include: Nile delta coastal lagoons opening to the Mediterranean (sites
1, 3, 5 and 7), a major sewage outfall off A
l
exandria city (site 2), the
R
iver Nile (site 4) and a major agricultural drain (site 6). Total phos-
phorus (P
t
o
t
) [1] and phosphorus forms [2] were determined on dried
suspended matter obtained by centrifugation.The five steps SEDEX
scheme was used to separate the following phosphorus pools:
exchangeable + loosely-bound (P
ex
); iron bound (P
f
e
); authigenic +
biogenic apatite + CaCO
3
bound P (P
a
u
); detrital apatite + other
remaining inorganic phases (P
d
e
t
) & organic (P
o
rg
) [2]. The method is
suitable for measuring concentrations <0.005 wt %P. The method wa
s
tested for effi
c
i
e
n
cy, specific
i
t
y, matrix effect and reproducibility.
Phosphate extracted from suspended matter was measured spectropho-
tometrically [3]. Simultaneously, suspended matter was tested for car-
bonate, organic content (Loss On Ignition) and chlorophyll
a.
Results and Discussion
The amount and type of water discharged from the different land-
based sources as well as its average TSM content are presented in
Table 1. The SE Mediterranean waters off the Egyptian coast receives
18.3x10
9
m
3
of fresh, brackish and wastewater (mainly sewage) annu-
ally from different landbased sources. About 20% of this amount are
discharged from the Nile while 28% are derived from Lake Manzalah,
the largest and most productive Nile Delta lake (Table 1).
Table 1. Amount and characteristics of water discharged to the
SE Mediterranean.
Location Landbased Discharge Points
L.
L.R.L.Gh.L.
MariutKBPSEdkuNileBurullusdrain
Manzalah
TSM(mg/l)26.95387.713811310531.5
POC/TSM %79683353322683
Chl
a(µg/l)4.28.62.31.10.90.810.9
Discharge*2.30.21.43.52.23.55.2
Water typeS+BSBFBBB
CO3/TSM %18315444625822
KBPS= Kay Bey Pump Station *x10
9
m
3
y
-1
S=SewageB=Brackish
F=Fresh
The concentrations of different particulate phosphorus species as
well as their contribution to P
tot
are presented in Table 2. P
ex
con-
tributed to about 2-22% of P
tot
. P
ex
concentrations are proportionally
related to particle size. Pex decrease gradually by increasing salinity
seawards from landbased sources.Variations in P
ex
concentrations
depend on alteration in redox conditions and adsorption capacity.
When semi-reducing conditions prevail, P
ex
reached maximum levels.
P
ex
is the most bioavalable fraction especially when phosphorus con-
centration in the water column is low.
Phosphorus associated with Ca appears either as detrital ?uoroap-
atite of igneous and metamorphic origin (P
det
) or other forms as bio-
genic skeletal debris and CaCO
3
incorporated phosphate (P
au
) [4]. P
det
showed a negative correlation with P
au
. Organic matter oxidation and
sulphide occurrence enhance CaCO
3
dissolution leading to low P
au
and P
det
contribution to P
tot
at locations 1 and 2. The extraction
method can not discriminate between apatite forms, e.g., fish bone
debris from igneous apatite-P. In the present study, at least 30-50% of
the suspended phosphorus pool represents an insoluble phase and is
ignored when looking for water column phosphorus enrichment.
Table 2. Average concentrations of particulate phosphorus frac-
tions (µmol g
-1
)
*Number between paranthesis is the % from the sum of species
The release of P associated with reducible iron oxide in the low
oxygen (<2-3.5 mg/l) and pH 6.8-7.3 bearing water, opposite to Lake
Mariut and sewage outfall, lowered the concentration of P
fe
. On the
other hand, ferric oxyhydroxide particles, formed under oxic condi-
tions, have a high adsorption capacity for phosphorus. The Nile water
characterized by fine-sized particles and high Fe levels showed the
maximum concentrations of P
fe
(5.2 ± 1.1 µmol g
-1
). The well-oxy-
genated waters (>5.5 mgl/l) and pH (8.1-8.3) render the P
fe
fraction to
be insoluble and non- reactive in the water column. Such a release
leads to highly eutrophic coastal areas as indicated by their elevated
chlorophyll
alevels (4.2 and 8.6 µg/L, respectively). The gradient of
P
fe
decline at the mixing zone with seawater was much more pro-
nounced than that of P
au
and P
det
.A high linear correlation (R
2
= 0.74,
p<0.001) appeared between total inorganic phosphorus and P
ex
indi-
Landbased Discharge Points
LakeKBPSLakeRiverLakeGharbLake
Form
Mariut
EdkuNileBurullusdrain
Manzalah
P
ex
2.16±0.70.65±0.30.4±0.20.34±0.20.6±0.10.±0.10.88±0.6
(22)(7)(6)(2)(7)(6)(6)
P
fe
0.3±0.10.3±0.91.8±0.35.2±1.12.69±0.32.66±0.21.67
0.5
(3)(3)(23)(35)(33)(31)(12)
P
au
1.9±0.42.1±1.12.7±0.93.6±1.72.1±1.11.9±1.64.3±0.9
(19)(22)(35)(24)(26)(22)(31)
P
det
1.21±0.70.8±0.31.1±0.61.7±1.01.9±0.72.4±1.60.9±0.4
(12)(8)(14)(12)(24)(28)(6)
P
org
4.31±2.815.84±1.11.66±1.33.94±1.20.8±0.21.15±0.96.37±2.09
‘44)(60)(22)(27)(10)(13)(45)
P
tot
10.5±2.110.3±3.67.8±1.914.8±3.38.3±1.58.7±1.114.8±2.8
