Macrophytes as refuge areas for zooplankton, a delicate balance set by littoral predacious macroinvertebrates

GONZALEZ SAGRARIO M. A. AND E. BALSEIRO

Punta del Este,

Congreso; Structure and function of world Shallow lakes; 2008

Universidad de la Republica, Facultad de Ciencias

<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0pt; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:595.45pt 841.7pt; margin:72.0pt 57.6pt 72.0pt 57.6pt; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> In shallow lakes, zooplankton seeks refuge in macrophytes when avoiding planktivorous fish. But, the effectiveness of macrophytes as refuge areas depends on various factors, for example, on the predators these plants host.  As a consequence, the benefit of escaping from pelagic predators must be balanced with the cost of confronting predators in the littoral zone. This study aimed to understand the use of macrophytes as refuge areas for zooplankton and the role of macroinvertebrates in the prevention of macrophyte refuge capacity. Methods. A study of diel horizontal (DHM) and vertical (DVM) migrations jointly with a field experiment were performed in a shallow lake (Los Padres Lake, Argentina) during summer in 2002 and 2003. To determine day and night distribution of zooplankton, macroinvertebrates and fish, 3 patches of the submerged macrophyte Ceratophyllum  demersum of similar size. In each macrophyte mat, a transect running from the central part of the stand to open water was performed, establishing three sampling points along them: 1- inner point (I) (at 1 meter from the edge of the macrophyte mat), 2- edge (E), and 3- open waters (OW) (at 6 meters from the edge of the macrophyte mat). At each point zooplankton, macroinvertebrates and fish were sampled during day (12:00 - 4:30 pm) and nigh (9:00 pm - 1:00 am) over two consecutive days. According with the zooplankton and macroinvertebrate distribution and abundance registered, a field experiment testing the effect of the predacious macroinvertebrate assemblage was carried out. Results & conclusions.  The daily pattern of distribution for zooplankton (median and large cladocerans and adults of calanoid copepods) showed a strong avoidance of C. demersum stands during the day and night. Instead, macroinvertebrates showed a strong association with macrophyte, occupying the inner and/or the edge parts of the vegetated patches during day and night, with potential zooplankton predators reaching very high abundances. The field experiment revealed that cladocera populations and adults of calanoid copepods declined (almost 80%) under a multiple predacious macroinvertebrate assemblage that mimic lake conditions with respect to treatments with having low abundance of predators or lacking them. The combination of macrophyte avoidance by zooplankton and high abundance of predators and impact on pelagic prey suggests that macroinvertebrates can cancel out the macrophyte refuge provided to zooplankton.