Case Studies - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 406

coastal lagoons 45 and agriculture is the main source of nitrogen. By the time
urban wastewater treatment plants were operational, agriculture started to change
from dry crop farming with low amounts of nitrogen fertilizers to intensive crop
irrigation with nitrogen overfertilization. During the dry agriculture period, nitro-
gen was the limiting nutrient for both benthic 42 and planktonic primary production
in the lagoon; nitrogen entered mainly via run-off and phosphorus entered through
urban sewage. 20
In the 1970s, the Mar Menor was oligotrophic, and primary productivity was
mainly benthic with the phanerogam Cymodocea nodosa as the main macrophyte.
During the early 1980s, after the enlargement of El Estacio, the bottoms were covered
by a mixed meadow of Cymodocea nodosa - Caulerpa prolifera , with a biomass of
about 280 g dw/m 2 ( Figure 9.3.13) . 21,41,42 By the early 1990s a dense bed of the
invasive macroalgae Caulerpa prolifera covered most of the bottom, restricting the
seaweed Cymodocea nodosa to patches in the shallowest areas. The high benthic
macrophyte biomass contrasted with the low phytoplanktonic density 46 and the
oligotrophy of the waters. 20 Based on data from the mid- to late 1980s, it was
estimated that 63.18% of the total primary production of the lagoon was due to
Caulerpa prolifera , 0.42% to Cymodocea nodosa , and 0.24% to photophilic algae,
with 11.62% due to microphytobenthos and 24.53% due to phytoplankton. 42
Changes in the trophic status of the lagoon waters can be seen by comparing two
extensive time series (weekly sampled), one for 1988 and the other for 1997. The time
series of 1988 showed that nitrate concentrations were low thoughout the year, in
contrast to the higher phosphate values. It also showed the seasonal variation in the
trophic state of the water due to different nutrient input regimes: nitrate mainly in
winter from run-off and phosphorus mainly in summer from urban sewage. While
nitrate concentration during 1988 ( Figure 9.3.14A) was always under 1

mol NO 3 − /l,
much higher concentrations occurred in 1997 ( Figure 9.3.14B) , particularly during
spring and summer (just at the harvest time when larger amounts of fertilizer are used
in the lagoon's watershed) entering mainly through the major watercourse (El Albujón)
due to the increase in the phreatic levels, as explained above. During 1997 higher
nitrate concentrations were usually found on the west coast of the lagoon, close to the
mouth of the main watercourses, while lower concentrations were found on the inner
coast of La Manga and the El Estacio Channel influence area ( Figure 9.3.15) , 47
suggesting that nitrate input was related to the agricultural activity.
Drastic changes in phosphate levels were also found between 1988 ( Figure 9.3.14C)
and 1997 ( Figure 9.3.14D) . The seasonal distribution of phosphate found in 1988 was
not evidenced in 1997, with much lower values probably due to the effect of wastewater
treatment plants. The spatial distribution of phosphorus in surface waters, on the other
hand, showed maximum concentrations close to the point sources (waste pipelines) of
the main wastewater treatment plants in La Ribera and Los Alcázares (in this case
through the Albujón watercourse) ( Figure 9.3.16) probably due to treatment plant
malfunctions. In 1997 the N:P ratio changed drastically as a consequence of higher
nitrate load and phosphate removal, with phosphate becoming the limiting factor for
planktonic productivity. As a consequence of changes in the nutrient input regimen, the
water column in the lagoon changed from moderately oligotrophic to relatively

µ