IS THE ZOOPLANKTON DECREASING? THE CASE OF 10 YEARS’STUDY
IN THE BALEARIC SEA (WESTERN MEDITERRANEAN): 1993-2003
Mª Luz Fernández de Puelles
Spanish Oceanographic Institute. Baleares Laboratory, Palma de Mallorca, Spain - * mluz.fernandez@ba.ieo.es
Abstract
The zooplankton off Mallorca has been studied for ten years (1993-2003),and related to the main physico-chemical properties of the
surface water. Data from a monitoring station at 75m depth were collected every 10 days, and zooplankton biomass and abundance were
estimated, and the main zooplankton groups identified. A significant decreasing trend was observed in both zooplankton indices with
increasing salinity. However, a decrease was observed during the warmest years (1994-1998), a linear temperature trend was found, mainly
due to the cooling of later years.
Key-words: Hydrography, zooplankton, time-series, Balearic Sea
Rapp. Comm. int. Mer Médit., 37,2004
354
Introduction
The Balearic Seaislocated in a boundary area between different
water masses in the Western Mediterranean (1), where strong
hydrographic annual variability is observed (2), related to changes in
the planktonic community (3). In addition, plankton exhibits great
variability over time, particularly at inter-annual level, in response to
climatic forcing. The mechanisms controlling that variation are still
far from understood and long-trend plankton studies are highly
recommended in the Mediterranean (4, 5). They emphasize the need
for assessing temporal trend to identify the causes of observed
changes. Our goal is to describe the results of a 10 years’study of the
zooplankton community and the environmental conditions in the
Balearic Sea.
Material and methods
From April 1993 to April 2003 sea water and plankton samples
were collected every 10 days from a neritic zone off Mallorca Island
(39º28´54”N; 2º25´57”E). To collect hydrographic data 5l Niskin
bottles were used at 0-15-25-50 and 75 m depth, and a CTD- Sbe19.
Zooplankton was collected by a Bongo Plankton net of 20 cm
diameter and 250 µm mesh by means of oblique haul (0-75 m). The
zooplankton samples were subdivided, the biomass samples were
frozen at -20ºC, and the composition samples were fixed in 5%
neutralised formaldehide. The laboratory analysis followed a protocol
detailed in previous study (6).
Results and discussion
Physical environment-Annual values of temperature based on
measurements made synchronously with salinity were registered, and
monthly data calculated. A clear seasonality was marked as a typical
thermic regime of these temperate latitudes (1). Interannual variability
was observed, with cooler winters during 1994, 1996 and 2000
(17.5ºC), and a warmer 1995 and 1998 (18.3ºC). During the study
period, annual mean values increased particularly during 1997 and
1998. Because of this variability, a linear trend was found during the
studied period. Although, an increase was observed during the spring
and summer from 1995 to 1998. In general, the minimum temperature
was measured in February-March (13.3ºC at 75m depth, 1994, 1996)
and the maximum in late August (27.4ºC in 1998). The highest
salinity values were registered during spring 1996 and 2000 (38.19),
and the lowest in 1995 and 1998 (37.10). These lower values were
related to recent Atlantic waters coming from the south (7). Higher
salinities, detected mainly during 1996 and 2000, were related to
northern Mediterranean waters (2). Due to the increase in annual
values from 1998, an upward trend was observed in the area (R
2
=0.56;
p<0.05).
Zooplankton– The annual cycle of the mesozooplankton showed
?uctuations, with peaks in winter and late spring. Biomass and
abundance of zooplankton were higher during spring 1996 and 2000,
almost entirely due to the presence of copepods. Significant
correlation was found between both indices (R
2
= 0.3; p<0.01). The
highest zooplankton abundance was related to cooler temperature
(17ºC, spring mean values) and higher salinity (>38), as noted by
previous studies (7) when northern Mediterranean waters were more
important in the Mallorca Channel. Lower salinity and warmer
temperature in 1995 and 1998 were related to more recent Atlantic
waters ?owing in (3, 2).
We observed a close relation between the zooplankton and the
surface water masses in the channel, particularly important during
spring. They relate a cooler and saltier waters with higher increase of
zooplankton. We confirm the considerable interannual variability in
physico-chemical properties of the Balearic Sea, driving the main
changes in zooplankton abundance. Even so, both indices - biomass
(R
2
=0.40; p<0.01) and abundance (R
2
=0.12; p<0.05) – decreased
during the 10 years’period. It would be useful to correlate the results
of our study with large-scale changes in zooplankton in other areas of
the Mediterranean and longer time-series.
Fig. 1. Annual zooplankton biomass (dashed line) and abundance (bar)
from April 1993 to April 2003.
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