Regulating mechanisms of calanoid copepods variability in the northern Adriatic Sea: Testing the roles of west-east salinity and phytoplankton gradients
Introduction
Copepods constitute the most important component of mesozooplankton in the Mediterranean coastal waters during the course of the year, mainly due to their high flexibility in adaption to fluctuating environmental conditions, seasonal succession of various species and ability to maintain a stable standing stock even in the presence of variable food sources (Mazzocchi and Ribera d'Alcalá, 1995). As the dominant organisms in any marine habitat, copepods are naturally very important to the overall grazing potential, especially in the more productive regions (Escribano et al., 2007, Takahashi et al., 2008).
Among copepod orders, calanoid copepods are traditionally targeted as consumers of phytoplankton and considered as the major link between primary production and higher trophic levels. Apart from the direct sequestration of phytoplankton biomass, ecological role of calanoids includes interactions with the microbial food web through predation on microzooplankton (Sherr and Sherr, 1988, Stoecker and Capuzzo, 1990), and contribution of dissolved material originating from excretion (Lampitt et al., 1990, Wassmann et al., 2000, Turner, 2002) and sloppy feeding (Møller and Nielsen, 2001, Møller et al., 2003).
In the open waters of northern and temperate zones, diversity and abundance of this copepod order are more directly modified by the physical environment (i.e. salinity, temperature, hydrodynamics) (Kiørboe et al., 1990, Möllmann et al., 2000), while trophic conditions are more determining to the tropical waters community (Paffenhöfer et al., 2003, Paffenhöfer et al., 2006).
Among the Adriatic subbasins, the northern Adriatic (further NA) is generally considered as the most productive part, where zooplankton is fueled by the high concentration of living and non-living particulate organic matter in addition to a high phytoplankton biomass sustained by the availability of nutrients (Guglielmo et al., 2002, Fonda Umani et al., 2005). The influence of the longitudinal north-south gradient on zooplankton communities in the Adriatic Sea is well documented; it has been shown that biomasses of all plankton groups show a strong decrease from the NA southwards, whereas a reverse gradient is evident for plankton diversity (Hure et al., 1980, Fonda Umani, 1996, Hure and Kršinić, 1998).
Significantly less attention is directed towards the west-east zooplankton variability, originating from the remarkable variations in environmental and biological conditions of the NA subbasin, due to meteorologically/climatically driven movements of water masses and freshwater nutrient contributions. Although NA waters have been sampled for zooplankton regularly during the last decades, most published data concerns either only microzooplankton (Kršinić, 1995, Lučić et al., 2003, Fonda Umani et al., 2005, Kršinić et al., 2007), or geographically confined location of the shallow and hydrographically highly variable Gulf of Trieste (Fonda Umani, 1996, Conversi et al., 2009, Conversi et al., 2010, Mozetič et al., 2012, Monti et al., 2012, Fonda Umani et al., 2012).
Due to the circulatory pattern that imports/exports water masses into the NA subbasin, the general salinity distribution at the transversal opposing ends of the subbasin are resulting from the variable inflow of Po River and its tributaries along the western side, and strength of the incoming Eastern Adriatic Current (EAC) that brings saltier waters along the eastern side (Degobbis et al., 2000, Supić and Ivančić, 2002). During the winter, when the water column is well mixed, the freshened Po River-influenced waters are generally confined to the western coast, whereas spreading over larger areas of the NA is observed during stratified water column periods in spring and summer (Franco and Michelato, 1992, Bernardi Aubry et al., 2006, Solidoro et al., 2009), or in the unstratified periods when circulation system favors spreading of nutrient enriched waters (Supić et al., 2012). Resulting from the usual regional dynamics and conditions is a west-to-east descending gradient of phytoplankton abundance (Totti et al., 2005).
It is expected that the coexistence of hydrologically and trophically distinct environments create a highly heterogeneous variety of habitats in the pelagic environment which are widely recognized as a major source of variability for plankton distribution and population dynamics (e.g. Paffenhöfer and Mazzocchi, 2003, Peralba and Mazzocchi, 2004). Our research is specifically oriented to the group of calanoid copepods, with the aim to assess regulating mechanisms of their population in the open waters of the NA, based on five years of species-specific abundances data matrix. By investigating calanoid species distributions and abundances as well as interannual cycles of dominant and/or frequent species in association with the salinity, temperature, freshwater discharge and phytoplankton data, we test the hypothesis that the differences in hydrological and biological properties at the opposing sides of the NA basin were significantly reflected in the variability of calanoid abundance and population structure in the 2002–2006 period.
Section snippets
Study area and sampling surveys
The study was conducted at two stations in the northern Adriatic (SJ107 – 45°2.9′N; 13°19′E, SJ108 – 44°45.4′N; 12°45′E) (Fig. 1) from 2002 to the end of 2006 aboard the R/V Vila Velebita (Center for Marine Research, Rovinj). Unless otherwise indicated, all samplings and measurements were performed at the standard depths: 0 m, 5 m, 10 m, 20 m below the sea level and 2 m above bottom (SJ107 – 35 m and SJ108 – 30 m).
Environmental parameters
Temperature was measured by protected reversing thermometers (Richter and Wiese,
Environmental conditions
The Po River discharge flow during 2002–2006 generally commenced by a period of the high flow, which was followed by a low flow period (Fig. 3A). Based on the deviation from the long-term averages, we can further distinguish between several periods summarized as: a period of intense discharge that lasted from May 2002 to January 2003 (during May–June, August–September and November–December 2002 large amounts of freshwater flowed into the NA, while during the remaining period, the flow was
Interannual variability of calanoid copepod community
Seasonal and interannual variability in the Po River flow are of great importance for the circulation and the trophic state of the NA basin (Degobbis et al., 2000, Socal et al., 2008). The historical average Po outflow for the period 1918–2006, based on daily data, is estimated at 1492 ± 999 m3 s−1, with flood peaks exceeding 5000 m3 s−1 and low flow of less than 500 m3 s−1 (source: ARPA Emilia-Romagna Report). Annual peaks are usually observed in May, mainly due to snow melting, and in
Conclusion
Based on the presented research results, we concluded that the observed salinity and phytoplankton gradients at the opposing sides of the NA basin were significantly reflected in the variability of calanoid abundances and community structure over the 2002–2006 period. While the magnitude and regularity of calanoid abundance peaks near the western coast were mostly driven by the Po River discharge pattern and phytoplankton cycle, this influence was diminished in the eastward direction, possibly
Acknowledgments
This work was supported by the Ministry of Science, Education and Sports of the Republic of Croatia (projects 275-0982705-3047 and “Project Jadran”). The authors thank all scientists and technicians that contributed to the collection of data, performing the measurements and sample analyses. The crew of the RV “Vila Velebita” is particularly appreciated for technical help in sampling during cruises. The critical comments of the reviewers are greatly appreciated.
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